After a timeout, how do you prove whether a retry created a duplicate write?

Discusses idempotency keys, request ids, commit time, and retry timing for duplicate API writes.

Duplicate writes after timeout are hard to explain because the client sees one failed attempt and one successful retry, while the server may have committed both. I look for request id, idempotency key, commit time, timeout time, retry time, and whether the same key was reused. What is the smallest timeline that proves whether the retry path is safe?