Why Payment APIs Double-Charge Customers and How Idempotency Keys Prevent It

A user who clicks an upgrade button twice during a network lag can trigger two separate card charges, creating duplicate ledger entries and corrupting billing state. The article explains that distributed web networks are unreliable, so a request can succeed on the server while the client sees a timeout and retries. It uses the Two Generals' Problem to illustrate why retries are unavoidable and cannot be treated as rare edge cases. Because HTTP POST is non-idempotent by design, repeated payment requests spawn new charges, unlike GET requests that can be safely repeated. The piece introduces idempotency keys as a way to bring retry safety to payment operations.