[pdonis]

Update: stickfigure basically answered this in another subthread: as I understand the answer, it is that you do check the idempotency key, but inside the DB transaction, so you have to start a new transaction on every request. If, inside the transaction, your idempotency key check shows that another request with that key was already received, you don't do anything to change the DB inside the transaction and just commit it as a no-op.