idempotency_key_reuse

HTTP 422. This Idempotency-Key was already used for a request with a DIFFERENT body. Keys identify one logical operation.

What to do: Generate a fresh key per operation; reuse a key only to retry the exact same request.

Every error is an RFC 9457 problem document - match on code (stable forever), show detail to humans, and quote instance when reporting an occurrence to support.