What is idempotency? Provide a case where an idempotent key is needed.

Idempotency is a property of an operation where multiple executions with the same input do not change the result and the system's state after the first call.

For example, if a request to create an order with a specific identifier is idempotent, repeated requests with the same identifier will not create duplicates but will return the result of the first call.

A case with an idempotency key is often encountered in payment system APIs. The client generates a unique key (for example, a UUID) when sending a payment and includes it with the request. If the request is repeated due to network issues, the server recognizes the payment as already processed by the key and will not trigger a double charge.