Glossary
Idempotency
Processing the same request twice has the same effect as once.
An operation is idempotent if performing it multiple times produces the same result as performing it once. For form integrations this matters because webhook delivery and network retries can send the same event more than once, and a non-idempotent handler would create duplicate records each time.
The standard technique is an idempotency key: the sender includes a unique event id, and the receiver records which ids it has already processed, ignoring repeats. The handler becomes safe to call again after a timeout or crash without double-charging, double-emailing or duplicating a response.
Idempotency is the property that makes reliable retries possible, and webhook signing is what makes those retries trustworthy — the two together are the difference between a robust integration and one that silently corrupts data under load. The webhooks guide covers implementing both.