Idempotency-Key header: any unique string up to 255 characters. Send a new key with each distinct create, and reuse the same key when you retry that create. Each endpoint’s reference page notes whether it accepts the header; endpoints that do not ignore it.
Retrying a keyed request within 24 hours returns the original response instead of creating a second resource. Replayed responses include Idempotency-Replayed: true. Reusing a key while the first request is still in progress returns 409. Reusing it on a different endpoint returns 422.