diff --git a/src/app/api/measurements/batch/route.ts b/src/app/api/measurements/batch/route.ts
index a0c2a7af1..16aea90fe 100644
--- a/src/app/api/measurements/batch/route.ts
+++ b/src/app/api/measurements/batch/route.ts
@@ -41,6 +41,13 @@ import {
 } from "@/lib/api-response";
 import { withIdempotency } from "@/lib/idempotency";
 import { mapAppleHealthEntry } from "@/lib/measurements/apple-health-mapping";
+import {
+  MEASURED_AT_TOLERANCE_MS,
+  isMergeableSource,
+  isSameReadingAcrossSource,
+  oppositeMergeSource,
+  type MergeCandidate,
+} from "@/lib/measurements/cross-source-merge";
 import { validateMeasurementRange } from "@/lib/validations/measurement";
 import { deviceTypeEnum } from "@/lib/validations/source-priority";
 import { checkRateLimit } from "@/lib/rate-limit";
@@ -287,6 +294,11 @@ async function postBatch(request: NextRequest): Promise<Response> {
   let insertedCount = 0;
   let updatedCount = 0;
   let duplicateCount = 0;
+  // v1.11.4 (iOS #2) — subset of `duplicateCount` collapsed by the
+  // MANUAL↔APPLE_HEALTH same-reading merge (as opposed to a plain
+  // composite-key duplicate). Surfaced as a dedicated wide-event count so
+  // an operator can see how often the standalone-pair mirror collapses.
+  let crossSourceMergedCount = 0;
 
   if (prepared.length > 0) {
     // Pre-flight duplicate detection so we can return per-entry status
@@ -320,6 +332,78 @@ async function postBatch(request: NextRequest): Promise<Response> {
       existing.map((row) => `${row.type}::${row.source}::${row.externalId}`),
     );
 
+    // v1.11.4 (iOS #2) — same-reading merge for the MANUAL ↔ APPLE_HEALTH
+    // mirror duplicate. When the iOS app logs a manual reading offline it
+    // mirrors that reading into HealthKit; on pairing, adopt-on-pair
+    // uploads it as MANUAL and HealthKit background sync independently
+    // re-ingests the mirrored sample as APPLE_HEALTH. The two carry
+    // different externalIds + sources, so the composite unique index lets
+    // both land — duplicating the whole hand-logged history on first pair.
+    // We collapse them at ingest: an incoming MANUAL / APPLE_HEALTH row is
+    // dropped as a `duplicate` when a same-reading row of the OPPOSITE
+    // source already exists (same type + value + measuredAt within a tight
+    // ±2 s window). `stats:*` cumulative rows are excluded — those are
+    // per-day aggregates the observer overwrites in place, never
+    // hand-entered point readings. See `cross-source-merge.ts` for the
+    // full rule + why first-physical-reading-wins (the value is identical
+    // between the two rows, so only the source label differs).
+    //
+    // Pull the opposite-source candidate rows in one widened query: for
+    // every mergeable, non-`stats:*` incoming row, look for an opposite-
+    // source row of the same type whose measuredAt falls inside the
+    // tolerance window. Compared in JS because the value match needs a
+    // float epsilon the SQL `IN` can't express.
+    const mergeProbes = prepared.filter(
+      (p) =>
+        isMergeableSource(p.row.source as string) &&
+        !isStatsExternalId(p.row.externalId as string),
+    );
+    const crossSourceCandidates: MergeCandidate[] = [];
+    if (mergeProbes.length > 0) {
+      const candidateRows = await prisma.measurement.findMany({
+        where: {
+          userId: user.id,
+          deletedAt: null,
+          OR: mergeProbes.map((p) => {
+            const measuredAt = p.row.measuredAt as Date;
+            return {
+              type: p.row.type as MeasurementType,
+              source: oppositeMergeSource(
+                p.row.source as "MANUAL" | "APPLE_HEALTH",
+              ),
+              measuredAt: {
+                gte: new Date(measuredAt.getTime() - MEASURED_AT_TOLERANCE_MS),
+                lte: new Date(measuredAt.getTime() + MEASURED_AT_TOLERANCE_MS),
+              },
+            };
+          }),
+        },
+        select: { type: true, source: true, value: true, measuredAt: true },
+      });
+      crossSourceCandidates.push(...candidateRows);
+    }
+
+    // Rows already chosen for insert earlier in THIS batch — so a MANUAL
+    // and an APPLE_HEALTH same-reading arriving in the SAME batch also
+    // collapse (the DB probe above only sees rows from prior batches).
+    const inBatchCandidates: MergeCandidate[] = [];
+    function hasSameReadingSibling(p: Prepared): boolean {
+      const incoming = {
+        type: p.row.type as MeasurementType,
+        source: p.row.source as string,
+        value: p.row.value as number,
+        measuredAt: p.row.measuredAt as Date,
+      };
+      if (!isMergeableSource(incoming.source)) return false;
+      for (const candidate of crossSourceCandidates) {
+        if (isSameReadingAcrossSource(incoming, candidate)) return true;
+      }
+      for (const candidate of inBatchCandidates) {
+        if (isSameReadingAcrossSource(incoming, candidate)) return true;
+      }
+      return false;
+    }
+
     const toInsert: Prisma.MeasurementCreateManyInput[] = [];
     const toOverwrite: Prepared[] = [];
     for (const p of prepared) {
@@ -336,9 +420,35 @@ async function postBatch(request: NextRequest): Promise<Response> {
           results[p.index] = { index: p.index, status: "duplicate" };
           duplicateCount += 1;
         }
+      } else if (
+        !isStatsExternalId(p.row.externalId as string) &&
+        hasSameReadingSibling(p)
+      ) {
+        // v1.11.4 (iOS #2) — cross-source same-reading collapse. The
+        // physical reading already exists under the opposite client
+        // source; drop this mirror copy. Status stays `duplicate` so the
+        // iOS sync cursor checkpoints past it exactly as it does for a
+        // composite-key duplicate; the `reason` distinguishes it for ops.
+        results[p.index] = {
+          index: p.index,
+          status: "duplicate",
+          reason: "cross_source_merge",
+        };
+        duplicateCount += 1;
+        crossSourceMergedCount += 1;
       } else {
         results[p.index] = { index: p.index, status: "inserted" };
         toInsert.push(p.row);
+        // Track this row so a same-reading sibling later in the SAME
+        // batch collapses against it.
+        if (isMergeableSource(p.row.source as string)) {
+          inBatchCandidates.push({
+            type: p.row.type as MeasurementType,
+            source: p.row.source as string,
+            value: p.row.value as number,
+            measuredAt: p.row.measuredAt as Date,
+          });
+        }
       }
     }
 
@@ -463,6 +573,22 @@ async function postBatch(request: NextRequest): Promise<Response> {
     });
   }
 
+  // v1.11.4 (iOS #2) — dedicated annotation for the MANUAL↔APPLE_HEALTH
+  // same-reading collapse so an operator can grep
+  // `measurement.batch.cross-source-merge` to confirm the standalone-pair
+  // mirror duplicate is being absorbed (a healthy first-pair flow). Only
+  // fires when at least one row was merged so the baseline ingest trace
+  // is unchanged for the common case.
+  if (crossSourceMergedCount > 0) {
+    annotate({
+      action: { name: "measurement.batch.cross-source-merge" },
+      meta: {
+        merged: crossSourceMergedCount,
+        processed: entries.length,
+      },
+    });
+  }
+
   // v1.4.25 W16c — kick off PR detection for this user. We always
   // enqueue when at least one row was written (or the batch had any
   // measurements to consider) so a single off-day reading still gets
diff --git a/src/lib/measurements/__tests__/cross-source-merge.test.ts b/src/lib/measurements/__tests__/cross-source-merge.test.ts
new file mode 100644
index 000000000..1ba162671
--- /dev/null
+++ b/src/lib/measurements/__tests__/cross-source-merge.test.ts
@@ -0,0 +1,185 @@
+/**
+ * v1.11.4 (iOS #2) — unit coverage for the MANUAL↔APPLE_HEALTH
+ * same-reading merge predicate. The route integration test exercises the
+ * end-to-end ingest path; this suite pins the pure matching rule:
+ *   - cross-source same reading → match
+ *   - same-source / server-source pair → no match (no over-merge)
+ *   - different value or out-of-tolerance timestamp → no match
+ */
+import { describe, expect, it } from "vitest";
+
+import {
+  MEASURED_AT_TOLERANCE_MS,
+  isMergeableSource,
+  isSameReadingAcrossSource,
+  measuredAtMatch,
+  oppositeMergeSource,
+  valuesMatch,
+  type MergeCandidate,
+} from "@/lib/measurements/cross-source-merge";
+import type { MeasurementType } from "@/generated/prisma/client";
+
+const T0 = new Date("2026-06-04T07:30:00.000Z");
+
+function candidate(over: Partial<MergeCandidate> = {}): MergeCandidate {
+  return {
+    type: "WEIGHT" as MeasurementType,
+    source: "MANUAL",
+    value: 81.4,
+    measuredAt: T0,
+    ...over,
+  };
+}
+
+function incoming(
+  over: Partial<{
+    type: MeasurementType;
+    source: string;
+    value: number;
+    measuredAt: Date;
+  }> = {},
+) {
+  return {
+    type: "WEIGHT" as MeasurementType,
+    source: "APPLE_HEALTH",
+    value: 81.4,
+    measuredAt: T0,
+    ...over,
+  };
+}
+
+describe("isMergeableSource", () => {
+  it("accepts only the two client-facing mirror sources", () => {
+    expect(isMergeableSource("MANUAL")).toBe(true);
+    expect(isMergeableSource("APPLE_HEALTH")).toBe(true);
+    expect(isMergeableSource("WITHINGS")).toBe(false);
+    expect(isMergeableSource("IMPORT")).toBe(false);
+    expect(isMergeableSource("COMPUTED")).toBe(false);
+    expect(isMergeableSource("WHOOP")).toBe(false);
+  });
+});
+
+describe("oppositeMergeSource", () => {
+  it("flips MANUAL ↔ APPLE_HEALTH", () => {
+    expect(oppositeMergeSource("MANUAL")).toBe("APPLE_HEALTH");
+    expect(oppositeMergeSource("APPLE_HEALTH")).toBe("MANUAL");
+  });
+});
+
+describe("valuesMatch", () => {
+  it("matches identical and sub-epsilon values", () => {
+    expect(valuesMatch(81.4, 81.4)).toBe(true);
+    expect(valuesMatch(81.4, 81.4 + 1e-12)).toBe(true);
+    expect(valuesMatch(0.97, 0.97)).toBe(true);
+  });
+
+  it("rejects genuinely different values", () => {
+    expect(valuesMatch(81.4, 81.5)).toBe(false);
+    expect(valuesMatch(120, 121)).toBe(false);
+  });
+});
+
+describe("measuredAtMatch", () => {
+  it("matches inside the ±tolerance window", () => {
+    expect(measuredAtMatch(T0, T0)).toBe(true);
+    expect(
+      measuredAtMatch(T0, new Date(T0.getTime() + MEASURED_AT_TOLERANCE_MS)),
+    ).toBe(true);
+    expect(
+      measuredAtMatch(T0, new Date(T0.getTime() - MEASURED_AT_TOLERANCE_MS)),
+    ).toBe(true);
+  });
+
+  it("rejects timestamps beyond the tolerance window", () => {
+    expect(
+      measuredAtMatch(
+        T0,
+        new Date(T0.getTime() + MEASURED_AT_TOLERANCE_MS + 1),
+      ),
+    ).toBe(false);
+    // A whole minute apart — two distinct readings, never the mirror pair.
+    expect(measuredAtMatch(T0, new Date(T0.getTime() + 60_000))).toBe(false);
+  });
+});
+
+describe("isSameReadingAcrossSource", () => {
+  it("matches a MANUAL candidate against an incoming APPLE_HEALTH mirror", () => {
+    expect(isSameReadingAcrossSource(incoming(), candidate())).toBe(true);
+  });
+
+  it("matches an APPLE_HEALTH candidate against an incoming MANUAL mirror", () => {
+    expect(
+      isSameReadingAcrossSource(
+        incoming({ source: "MANUAL" }),
+        candidate({ source: "APPLE_HEALTH" }),
+      ),
+    ).toBe(true);
+  });
+
+  it("matches within the timestamp tolerance", () => {
+    expect(
+      isSameReadingAcrossSource(
+        incoming({ measuredAt: new Date(T0.getTime() + 1_500) }),
+        candidate(),
+      ),
+    ).toBe(true);
+  });
+
+  it("does NOT match a same-source pair (no over-merge)", () => {
+    // Two APPLE_HEALTH readings — keep the existing per-source contract.
+    expect(
+      isSameReadingAcrossSource(
+        incoming({ source: "APPLE_HEALTH" }),
+        candidate({ source: "APPLE_HEALTH" }),
+      ),
+    ).toBe(false);
+    // Two MANUAL readings.
+    expect(
+      isSameReadingAcrossSource(
+        incoming({ source: "MANUAL" }),
+        candidate({ source: "MANUAL" }),
+      ),
+    ).toBe(false);
+  });
+
+  it("does NOT match when a server-owned source is involved", () => {
+    expect(
+      isSameReadingAcrossSource(
+        incoming({ source: "APPLE_HEALTH" }),
+        candidate({ source: "WITHINGS" }),
+      ),
+    ).toBe(false);
+    // Incoming WITHINGS could never reach this route, but the predicate
+    // must still refuse it.
+    expect(
+      isSameReadingAcrossSource(
+        incoming({ source: "WITHINGS" }),
+        candidate({ source: "APPLE_HEALTH" }),
+      ),
+    ).toBe(false);
+  });
+
+  it("does NOT match a different measurement type", () => {
+    expect(
+      isSameReadingAcrossSource(
+        incoming({ type: "PULSE" as MeasurementType }),
+        candidate({ type: "WEIGHT" as MeasurementType }),
+      ),
+    ).toBe(false);
+  });
+
+  it("does NOT match a different value (two real readings, same minute)", () => {
+    expect(
+      isSameReadingAcrossSource(incoming({ value: 81.4 }), candidate({ value: 82.0 })),
+    ).toBe(false);
+  });
+
+  it("does NOT match a timestamp beyond the tolerance window", () => {
+    expect(
+      isSameReadingAcrossSource(
+        incoming({ measuredAt: new Date(T0.getTime() + 60_000) }),
+        candidate(),
+      ),
+    ).toBe(false);
+  });
+});
diff --git a/src/lib/measurements/cross-source-merge.ts b/src/lib/measurements/cross-source-merge.ts
new file mode 100644
index 000000000..00dbe612d
--- /dev/null
+++ b/src/lib/measurements/cross-source-merge.ts
@@ -0,0 +1,151 @@
+/**
+ * v1.11.4 (iOS #2) — same-reading merge for the MANUAL ↔ APPLE_HEALTH
+ * mirror duplicate.
+ *
+ * ## The problem
+ * When the iOS app runs standalone/offline and the user logs a MANUAL
+ * measurement, iOS mirrors that reading into HealthKit too. On pairing a
+ * server, two independent upload paths then reach
+ * `POST /api/measurements/batch` for the SAME physical reading:
+ *   1. adopt-on-pair backfill uploads it as `source = MANUAL`
+ *      (externalId = the app's local id), and
+ *   2. HealthKit background sync re-ingests the mirrored sample and
+ *      uploads it as `source = APPLE_HEALTH` (externalId = HKSample.uuid).
+ *
+ * The two carry different `externalId`s and different `source`s, so the
+ * `(userId, type, source, externalId)` unique index treats them as two
+ * distinct rows — duplicating the whole manually-logged history on first
+ * pair.
+ *
+ * ## The rule
+ * Treat a MANUAL row and an APPLE_HEALTH row as the SAME physical
+ * reading when they share:
+ *   - the same `userId` (multi-tenant scoping — always narrowed), AND
+ *   - the same `type`, AND
+ *   - the same `value` (within a tiny float epsilon — both rows are the
+ *     same hand-entered number run through the identical
+ *     `mapAppleHealthEntry` conversion, so equality is exact in practice;
+ *     the epsilon only guards against sub-ulp drift), AND
+ *   - a `measuredAt` within `MEASURED_AT_TOLERANCE_MS` of each other.
+ *
+ * The merge is deliberately ONE-SIDED in scope: it only fires for the
+ * MANUAL ↔ APPLE_HEALTH cross-source pair. MANUAL ↔ MANUAL,
+ * APPLE_HEALTH ↔ APPLE_HEALTH, and any pair involving a server-owned
+ * source (WITHINGS / IMPORT / COMPUTED / WHOOP) keep the existing
+ * first-write-wins / per-source contracts untouched. Two genuinely
+ * distinct readings at the same minute from two devices therefore never
+ * collapse — they are same-source or involve a server source, both of
+ * which fall outside this rule.
+ *
+ * ## Ingest-time, first-physical-reading-wins
+ * The collapse runs at INGEST time, not read time: read-time dedup would
+ * have to live in the rollup tier and every analytics path forever.
+ * Ingest-time keeps the storage clean and the rule contained.
+ *
+ * Because the two uploads can arrive in either order and minutes apart
+ * in SEPARATE batches, the collapse compares each incoming row against
+ * the rows already PERSISTED for the user (cross-batch) as well as the
+ * rows already chosen for insert earlier in the SAME batch.
+ *
+ * The winner is whichever physical reading landed first. The `value` is
+ * identical between the two rows by construction, so the only difference
+ * the surviving row carries is its `source` label — the numeric series
+ * the user sees is the same either way. A source-priority-aware REPLACE
+ * (overwrite the loser's source/row with the higher-ranked source) was
+ * considered and rejected for a patch: it would require mutating or
+ * re-sourcing an existing row (rollup re-source + sync-version churn) for
+ * zero change to the displayed value. First-physical-reading-wins is the
+ * natural extension of the endpoint's existing first-write-wins
+ * philosophy across the MANUAL↔APPLE_HEALTH boundary.
+ *
+ * `stats:*` cumulative externalIds are explicitly OUT of scope — those
+ * are per-day aggregate rows the iOS observer overwrites in place
+ * (issue #213), never hand-entered point readings, so the cross-source
+ * mirror duplicate cannot arise for them.
+ */
+import type { MeasurementType } from "@/generated/prisma/client";
+
+/**
+ * Timestamp tolerance for the same-reading match. A reading the user
+ * hand-enters and the app mirrors to HealthKit shares the same instant;
+ * the HealthKit re-ingest may round to the second or carry a sub-second
+ * drift, so a tight ±2 s window absorbs that without any chance of
+ * catching a separate genuine reading — a user does not hand-enter two
+ * distinct readings of the same type within two seconds.
+ */
+export const MEASURED_AT_TOLERANCE_MS = 2_000;
+
+/**
+ * Relative epsilon for the value match. Both rows run the same
+ * `convertToDbUnit` over the same source number, so the stored floats
+ * are bit-identical in practice; the epsilon is a defensive guard
+ * against any future conversion that introduces sub-ulp rounding. Scaled
+ * to the magnitude of the value so it stays meaningful for both a
+ * 0.97-fraction SpO2 and a 12 000-step count.
+ */
+const VALUE_RELATIVE_EPSILON = 1e-9;
+
+/** The two client-facing sources that can mirror the same reading. */
+type MergeableSource = "MANUAL" | "APPLE_HEALTH";
+
+/** The opposite member of the MANUAL ↔ APPLE_HEALTH pair. */
+export function oppositeMergeSource(source: MergeableSource): MergeableSource {
+  return source === "MANUAL" ? "APPLE_HEALTH" : "MANUAL";
+}
+
+/**
+ * True when `source` participates in the cross-source merge. Only the
+ * two client-facing mirror sources qualify; server-owned sources are
+ * untouched.
+ */
+export function isMergeableSource(source: string): source is MergeableSource {
+  return source === "MANUAL" || source === "APPLE_HEALTH";
+}
+
+/** Are two values the same reading within the relative epsilon? */
+export function valuesMatch(a: number, b: number): boolean {
+  if (a === b) return true;
+  const scale = Math.max(Math.abs(a), Math.abs(b), 1);
+  return Math.abs(a - b) <= VALUE_RELATIVE_EPSILON * scale;
+}
+
+/** Are two instants within the same-reading tolerance window? */
+export function measuredAtMatch(a: Date, b: Date): boolean {
+  return Math.abs(a.getTime() - b.getTime()) <= MEASURED_AT_TOLERANCE_MS;
+}
+
+/**
+ * A candidate same-reading row — either already persisted for the user
+ * or already chosen for insert earlier in the same batch. Carries only
+ * the fields the match needs.
+ */
+export interface MergeCandidate {
+  type: MeasurementType;
+  source: string;
+  value: number;
+  measuredAt: Date;
+}
+
+/**
+ * Does `candidate` represent the same physical reading as an incoming
+ * row of `(type, incomingSource, value, measuredAt)`, under the
+ * cross-source MANUAL↔APPLE_HEALTH rule? Requires:
+ *   - the incoming row's source is mergeable,
+ *   - the candidate's source is the OPPOSITE mergeable source,
+ *   - same type, value (±epsilon), measuredAt (±tolerance).
+ */
+export function isSameReadingAcrossSource(
+  incoming: {
+    type: MeasurementType;
+    source: string;
+    value: number;
+    measuredAt: Date;
+  },
+  candidate: MergeCandidate,
+): boolean {
+  if (!isMergeableSource(incoming.source)) return false;
+  if (candidate.source !== oppositeMergeSource(incoming.source)) return false;
+  if (candidate.type !== incoming.type) return false;
+  if (!valuesMatch(candidate.value, incoming.value)) return false;
+  return measuredAtMatch(candidate.measuredAt, incoming.measuredAt);
+}
diff --git a/tests/integration/measurements-batch.test.ts b/tests/integration/measurements-batch.test.ts
index 81fabf5a8..eb498ad16 100644
--- a/tests/integration/measurements-batch.test.ts
+++ b/tests/integration/measurements-batch.test.ts
@@ -764,6 +764,11 @@ describe("POST /api/measurements/batch (real Postgres)", () => {
     it("treats the same externalId under different sources as two distinct rows", async () => {
       const { POST } = await import("@/app/api/measurements/batch/route");
 
+      // The two rows share an externalId but carry DIFFERENT values +
+      // timestamps so they are genuinely distinct readings — this asserts
+      // that `source` is part of the composite dedup key, without tripping
+      // the v1.11.4 cross-source same-reading merge (which only collapses a
+      // MANUAL + APPLE_HEALTH pair that share value + measuredAt).
       const sharedExternalId = "uuid-source-collision-001";
       const response = await POST(
         makeRequest({
@@ -779,10 +784,10 @@ describe("POST /api/measurements/batch (real Postgres)", () => {
             },
             {
               hkIdentifier: "HKQuantityTypeIdentifierBodyMass",
-              value: 81.0,
+              value: 79.5,
               unit: "kg",
-              startDate: "2026-05-09T07:30:00.000Z",
-              endDate: "2026-05-09T07:30:00.000Z",
+              startDate: "2026-05-09T19:30:00.000Z",
+              endDate: "2026-05-09T19:30:00.000Z",
               externalId: sharedExternalId,
               source: "MANUAL",
             },
@@ -806,6 +811,301 @@ describe("POST /api/measurements/batch (real Postgres)", () => {
       );
     });
 
+    // v1.11.4 (iOS #2) — same-reading merge for the MANUAL↔APPLE_HEALTH
+    // mirror duplicate. A standalone manual reading mirrors into HealthKit;
+    // on pair, adopt-on-pair uploads it as MANUAL and HealthKit background
+    // sync re-ingests the mirror as APPLE_HEALTH with a different
+    // externalId. The composite unique index would let both land; the
+    // cross-source collapse drops the second copy as a `duplicate`.
+    describe("cross-source same-reading merge (iOS #2)", () => {
+      const weightEntry = (over: Partial<BatchEntryFixture>): BatchEntryFixture => ({
+        hkIdentifier: "HKQuantityTypeIdentifierBodyMass",
+        value: 81.4,
+        unit: "kg",
+        startDate: "2026-06-04T07:30:00.000Z",
+        endDate: "2026-06-04T07:30:00.000Z",
+        externalId: "uuid-merge-default",
+        ...over,
+      });
+
+      it("collapses MANUAL then APPLE_HEALTH same reading into one row (separate batches)", async () => {
+        const { POST } = await import("@/app/api/measurements/batch/route");
+
+        // Adopt-on-pair MANUAL upload first.
+        const first = await POST(
+          makeRequest({
+            entries: [
+              weightEntry({ externalId: "local-manual-001", source: "MANUAL" }),
+            ],
+          }),
+        );
+        expect(first.status).toBe(200);
+        expect(((await first.json()) as { data: { inserted: number } }).data.inserted).toBe(1);
+
+        // HealthKit background sync re-ingests the same physical reading
+        // as APPLE_HEALTH with a different (HKSample.uuid) externalId.
+        const second = await POST(
+          makeRequest({
+            entries: [
+              weightEntry({ externalId: "hk-uuid-001", source: "APPLE_HEALTH" }),
+            ],
+          }),
+        );
+        expect(second.status).toBe(200);
+        const secondJson = (await second.json()) as {
+          data: {
+            inserted: number;
+            duplicates: number;
+            entries: Array<{ status: string; reason?: string }>;
+          };
+        };
+        expect(secondJson.data.inserted).toBe(0);
+        expect(secondJson.data.duplicates).toBe(1);
+        expect(secondJson.data.entries[0]?.status).toBe("duplicate");
+        expect(secondJson.data.entries[0]?.reason).toBe("cross_source_merge");
+
+        const stored = await getPrismaClient().measurement.findMany({
+          where: { userId: TEST_USER_ID, type: "WEIGHT" },
+        });
+        expect(stored).toHaveLength(1);
+        expect(stored[0]!.source).toBe("MANUAL");
+      });
+
+      it("collapses regardless of arrival order (APPLE_HEALTH then MANUAL)", async () => {
+        const { POST } = await import("@/app/api/measurements/batch/route");
+
+        const first = await POST(
+          makeRequest({
+            entries: [
+              weightEntry({ externalId: "hk-uuid-002", source: "APPLE_HEALTH" }),
+            ],
+          }),
+        );
+        expect(first.status).toBe(200);
+
+        const second = await POST(
+          makeRequest({
+            entries: [
+              weightEntry({ externalId: "local-manual-002", source: "MANUAL" }),
+            ],
+          }),
+        );
+        expect(second.status).toBe(200);
+        const secondJson = (await second.json()) as {
+          data: { inserted: number; duplicates: number };
+        };
+        expect(secondJson.data.inserted).toBe(0);
+        expect(secondJson.data.duplicates).toBe(1);
+
+        const stored = await getPrismaClient().measurement.findMany({
+          where: { userId: TEST_USER_ID, type: "WEIGHT" },
+        });
+        expect(stored).toHaveLength(1);
+        expect(stored[0]!.source).toBe("APPLE_HEALTH");
+      });
+
+      it("collapses a MANUAL + APPLE_HEALTH mirror arriving in the SAME batch", async () => {
+        const { POST } = await import("@/app/api/measurements/batch/route");
+
+        const response = await POST(
+          makeRequest({
+            entries: [
+              weightEntry({ externalId: "local-manual-003", source: "MANUAL" }),
+              weightEntry({ externalId: "hk-uuid-003", source: "APPLE_HEALTH" }),
+            ],
+          }),
+        );
+        expect(response.status).toBe(200);
+        const json = (await response.json()) as {
+          data: { inserted: number; duplicates: number };
+        };
+        expect(json.data.inserted).toBe(1);
+        expect(json.data.duplicates).toBe(1);
+
+        const stored = await getPrismaClient().measurement.findMany({
+          where: { userId: TEST_USER_ID, type: "WEIGHT" },
+        });
+        expect(stored).toHaveLength(1);
+      });
+
+      it("matches within the ±2s tolerance window", async () => {
+        const { POST } = await import("@/app/api/measurements/batch/route");
+
+        await POST(
+          makeRequest({
+            entries: [
+              weightEntry({
+                externalId: "local-manual-tol",
+                source: "MANUAL",
+                startDate: "2026-06-04T07:30:00.000Z",
+                endDate: "2026-06-04T07:30:00.000Z",
+              }),
+            ],
+          }),
+        );
+
+        // HK mirror ingested with a 1s sub-second drift on the same reading.
+        const second = await POST(
+          makeRequest({
+            entries: [
+              weightEntry({
+                externalId: "hk-uuid-tol",
+                source: "APPLE_HEALTH",
+                startDate: "2026-06-04T07:30:01.000Z",
+                endDate: "2026-06-04T07:30:01.000Z",
+              }),
+            ],
+          }),
+        );
+        expect(second.status).toBe(200);
+        const secondJson = (await second.json()) as {
+          data: { inserted: number; duplicates: number };
+        };
+        expect(secondJson.data.inserted).toBe(0);
+        expect(secondJson.data.duplicates).toBe(1);
+
+        const stored = await getPrismaClient().measurement.findMany({
+          where: { userId: TEST_USER_ID, type: "WEIGHT" },
+        });
+        expect(stored).toHaveLength(1);
+      });
+
+      it("does NOT merge different values (two real readings same minute)", async () => {
+        const { POST } = await import("@/app/api/measurements/batch/route");
+
+        await POST(
+          makeRequest({
+            entries: [
+              weightEntry({
+                externalId: "local-manual-distinct",
+                source: "MANUAL",
+                value: 81.4,
+              }),
+            ],
+          }),
+        );
+
+        const second = await POST(
+          makeRequest({
+            entries: [
+              weightEntry({
+                externalId: "hk-uuid-distinct",
+                source: "APPLE_HEALTH",
+                value: 82.0,
+              }),
+            ],
+          }),
+        );
+        expect(second.status).toBe(200);
+        const secondJson = (await second.json()) as {
+          data: { inserted: number; duplicates: number };
+        };
+        // Different value → genuinely distinct reading, both rows survive.
+        expect(secondJson.data.inserted).toBe(1);
+        expect(secondJson.data.duplicates).toBe(0);
+
+        const stored = await getPrismaClient().measurement.findMany({
+          where: { userId: TEST_USER_ID, type: "WEIGHT" },
+        });
+        expect(stored).toHaveLength(2);
+      });
+
+      it("does NOT merge timestamps beyond the tolerance window", async () => {
+        const { POST } = await import("@/app/api/measurements/batch/route");
+
+        await POST(
+          makeRequest({
+            entries: [
+              weightEntry({
+                externalId: "local-manual-far",
+                source: "MANUAL",
+                startDate: "2026-06-04T07:30:00.000Z",
+                endDate: "2026-06-04T07:30:00.000Z",
+              }),
+            ],
+          }),
+        );
+
+        // A full minute later — distinct reading, not the mirror pair.
+        const second = await POST(
+          makeRequest({
+            entries: [
+              weightEntry({
+                externalId: "hk-uuid-far",
+                source: "APPLE_HEALTH",
+                startDate: "2026-06-04T07:31:00.000Z",
+                endDate: "2026-06-04T07:31:00.000Z",
+              }),
+            ],
+          }),
+        );
+        expect(second.status).toBe(200);
+        const secondJson = (await second.json()) as {
+          data: { inserted: number; duplicates: number };
+        };
+        expect(secondJson.data.inserted).toBe(1);
+        expect(secondJson.data.duplicates).toBe(0);
+
+        const stored = await getPrismaClient().measurement.findMany({
+          where: { userId: TEST_USER_ID, type: "WEIGHT" },
+        });
+        expect(stored).toHaveLength(2);
+      });
+
+      it("leaves the stats:* overwrite contract untouched (no cross-source collapse)", async () => {
+        const { POST } = await import("@/app/api/measurements/batch/route");
+
+        const externalId = "stats:HKQuantityTypeIdentifierStepCount:2026-06-04";
+        // A MANUAL same-day-total then an APPLE_HEALTH re-post must NOT
+        // collapse — stats:* rows are per-day aggregates, excluded from the
+        // merge. They are distinct (type, source) rows and each keeps its
+        // own overwrite lane.
+        await POST(
+          makeRequest({
+            entries: [
+              {
+                hkIdentifier: "HKQuantityTypeIdentifierStepCount",
+                value: 4000,
+                unit: "count",
+                startDate: "2026-06-04T00:00:00.000Z",
+                endDate: "2026-06-04T12:00:00.000Z",
+                externalId,
+                source: "MANUAL",
+              },
+            ],
+          }),
+        );
+
+        const second = await POST(
+          makeRequest({
+            entries: [
+              {
+                hkIdentifier: "HKQuantityTypeIdentifierStepCount",
+                value: 4000,
+                unit: "count",
+                startDate: "2026-06-04T00:00:00.000Z",
+                endDate: "2026-06-04T12:00:00.000Z",
+                externalId,
+                source: "APPLE_HEALTH",
+              },
+            ],
+          }),
+        );
+        expect(second.status).toBe(200);
+        const secondJson = (await second.json()) as {
+          data: { inserted: number; duplicates: number };
+        };
+        // Different source on a stats:* externalId → distinct row, inserted.
+        expect(secondJson.data.inserted).toBe(1);
+        expect(secondJson.data.duplicates).toBe(0);
+
+        const stored = await getPrismaClient().measurement.findMany({
+          where: { userId: TEST_USER_ID, externalId },
+        });
+        expect(stored).toHaveLength(2);
+      });
+    });
+
     it("rejects an out-of-range source value with 422", async () => {
       const { POST } = await import("@/app/api/measurements/batch/route");