diff --git a/src/lib/api/__tests__/use-idempotent-post.test.tsx b/src/lib/api/__tests__/use-idempotent-post.test.tsx
index cd307c480..f025086c4 100644
--- a/src/lib/api/__tests__/use-idempotent-post.test.tsx
+++ b/src/lib/api/__tests__/use-idempotent-post.test.tsx
@@ -63,6 +63,53 @@ describe("useIdempotentPost", () => {
     expect(keyOf(1)).not.toBe(keyOf(0));
   });
 
+  it("takes a new key once the server refused the entry", async () => {
+    // The refusal is stored under the key, so the corrected retry under it
+    // ran unprotected and an unchanged one replayed the refusal.
+    fetchMock
+      .mockResolvedValueOnce(
+        new Response(
+          JSON.stringify({ data: null, error: "Name taken", meta: {} }),
+          { status: 409, headers: { "Content-Type": "application/json" } },
+        ),
+      )
+      .mockResolvedValueOnce(stored());
+    const post = openForm();
+
+    await expect(
+      post("/api/custom-metrics", { name: "Steps" }),
+    ).rejects.toThrow();
+    await post("/api/custom-metrics", { name: "Steps" });
+
+    expect(keyOf(1)).not.toBe(keyOf(0));
+  });
+
+  it("keeps the key while the first request is still running", async () => {
+    fetchMock
+      .mockResolvedValueOnce(
+        new Response(
+          JSON.stringify({
+            data: null,
+            error: "A request with this Idempotency-Key is already in progress",
+          }),
+          {
+            status: 409,
+            headers: {
+              "Content-Type": "application/json",
+              "X-Idempotent-Replay": "false",
+            },
+          },
+        ),
+      )
+      .mockResolvedValueOnce(stored());
+    const post = openForm();
+
+    await expect(post("/api/mood-entries", { mood: "GOOD" })).rejects.toThrow();
+    await post("/api/mood-entries", { mood: "GOOD" });
+
+    expect(keyOf(1)).toBe(keyOf(0));
+  });
+
   it("keeps a header the caller set", async () => {
     fetchMock.mockResolvedValueOnce(stored());
     const post = openForm();
diff --git a/src/lib/api/api-fetch.ts b/src/lib/api/api-fetch.ts
index 7d180aaea..5e23d76c2 100644
--- a/src/lib/api/api-fetch.ts
+++ b/src/lib/api/api-fetch.ts
@@ -79,6 +79,12 @@ export class ApiError extends Error {
    * so a form can mark the one the server refused. Empty when it named none.
    */
   readonly issuePaths: readonly string[];
+  /**
+   * The idempotency layer's "a request under this key is still running" (a
+   * 409 with `X-Idempotent-Replay: false`): nothing was decided, and a retry
+   * under the same key replays the first request's answer.
+   */
+  readonly inFlight: boolean;
 
   constructor(
     message: string,
@@ -86,12 +92,14 @@ export class ApiError extends Error {
     meta?: Record<string, unknown>,
     requestId?: string | null,
     issuePaths: readonly string[] = [],
+    inFlight = false,
   ) {
     super(message);
     this.name = "ApiError";
     this.status = status;
     this.meta = meta;
     this.issuePaths = issuePaths;
+    this.inFlight = inFlight;
     const fromMeta =
       typeof meta?.requestId === "string" && meta.requestId.length > 0
         ? meta.requestId
@@ -133,6 +141,7 @@ export async function throwApiError(res: Response): Promise<never> {
     meta,
     res.headers.get("x-request-id"),
     issuePaths,
+    res.status === 409 && res.headers.get("x-idempotent-replay") === "false",
   );
   // A server-side failure is worth an id in the session's support info,
   // whether or not the call site ends up showing it.
diff --git a/src/lib/api/use-idempotent-post.ts b/src/lib/api/use-idempotent-post.ts
index 4bd1ac40a..28aae8453 100644
--- a/src/lib/api/use-idempotent-post.ts
+++ b/src/lib/api/use-idempotent-post.ts
@@ -2,19 +2,23 @@
 
 import { useRef } from "react";
 
-import { apiPost } from "@/lib/api/api-fetch";
+import { ApiError, apiPost } from "@/lib/api/api-fetch";
 import { randomId } from "@/lib/random-id";
 
 /**
- * `apiPost` for a create form, under one `Idempotency-Key` until a save lands.
+ * `apiPost` for a create form, under one `Idempotency-Key` until an answer
+ * arrives.
  *
  * The transport mints a fresh key per call, which covers a replayed request
  * but not a second tap on Save: when the first answer never arrived (the 15 s
  * timeout on a weak signal) the entry is already stored, and the retry is a
  * new call with a new key, so the server stored it twice. Every submit
- * through this function sends the same key until one succeeds, so the retry
- * replays the first answer instead; a retry with a changed body is a
- * different request and runs as one. The next entry gets a new key.
+ * through this function sends the same key until the server answers, so the
+ * retry replays the first answer instead. An answer settles the key, a
+ * refusal included: the server keeps a refusal under its key for a day, so a
+ * corrected retry under the old key was never protected and an unchanged one
+ * replayed a refusal that no longer held. Only the answer that the first
+ * request is still running keeps it. The next entry gets a new key.
  */
 export function useIdempotentPost() {
   const key = useRef<string | null>(null);
@@ -26,8 +30,13 @@ export function useIdempotentPost() {
     key.current ??= randomId();
     const headers = new Headers(init?.headers);
     headers.set("Idempotency-Key", key.current);
-    const result = await apiPost<T>(path, body, { ...init, headers });
-    key.current = null;
-    return result;
+    try {
+      const result = await apiPost<T>(path, body, { ...init, headers });
+      key.current = null;
+      return result;
+    } catch (error) {
+      if (error instanceof ApiError && !error.inFlight) key.current = null;
+      throw error;
+    }
   };
 }