How would you prevent a checkout retry from creating two orders?

Accept an idempotency key representing the user’s intended checkout operation and store it with a request fingerprint and result under a uniqueness constraint. Atomically associate the key with order creation so simultaneous retries cannot both succeed independently. Return the existing outcome for a matching retry and reject reuse with different inputs. Coordinate payment-provider idempotency and handle partial failures explicitly. Disabling the button improves the interface but does not prevent network retries, duplicate tabs, or repeated requests sent directly to the API.