# Idempotency

> Send an Idempotency-Key on writes so a retry never creates something twice.

Send an `Idempotency-Key` header on write requests, so a retry never creates something twice. It's required when you prepare a payment run or submit an invoice.

Use a unique value for each action, such as a UUID. If you repeat a request with the same key and body within 24 hours, you get the original response back. The same key with a different body returns an error.