A few years ago, I spent about a month cleaning up duplicate charges. Every service involved already had idempotency "handled": the client sends an Idempotency-Key header, we store the response under it, and if the key comes back we replay the stored response. Textbook. And it still double-charged people. What finally clicked for me is that the key is just a name for the request. It doesn't do anything on its own. Whatever safety you have comes from the code around it, and that code is a smal...