What does an Idempotent API mean?

An idempotent API endpoint produces the same result whether you call it once or ten times, because repeated identical requests don't create duplicate effects. It's the property that stops a double-click from becoming a double-charge.

The problem it solves is that networks fail mid-request. If a payment call times out, the client can't know whether the server processed it, so it retries. Without idempotency that retry might charge the card twice, and with it, retrying is always safe. The standard implementation is an idempotency key: the client attaches a unique ID, the server records the outcome of the first execution under that key, and any repeat with the same key gets the stored response instead of re-executing. Some HTTP methods are idempotent by definition, since deleting something twice still leaves it deleted, while POST is not, which is exactly why payment APIs bolt keys onto it. Every "do not press submit twice" warning on a checkout page is a system admitting it skipped this.

An elevator button. Press it once or press it seven times while sighing, and one elevator comes, because the request registers once and every repeat maps onto the same outcome. An elevator that dispatched seven cars would be a payment endpoint without idempotency.