What happened
A retry arrived while the first request sent with the sameIdempotency-Key was still running, typically because the client timed out sooner than the server finished. The retry did not run, so the write still happens at most once.
How to fix
WaitRetry-After seconds and retry with the same key. Once the first request finishes, a retry gets its response back with Idempotent-Replayed: true, or runs again if the first request failed. See Idempotent requests.