Sequential retries can pass while concurrent requests sharing one Idempotency-Key still produce more than one logical result. A retry loop sends a request, waits for the response, then sends the next one, so the second request finds the stored result. A burst does not wait, and both requests do the work. Sequential retries: PASS Concurrent duplicates: FAIL Same Idempotency-Key ??multiple logical results The usual implementation looks like this: check key ??do work ...