Idempotency

Send Idempotency-Key (any unique string up to 255 characters) with a write, and retrying it is safe: for 24 hours the same key with the same request returns the first answer with Idempotent-Replayed: true. The same key with a different request answers 422 idempotency_mismatch; while the first is still running, 409 idempotency_in_progress.

It is required on operations that reach people, spend or publish (for example publishing a page).