In what case is an idempotency key needed?

An idempotency key is needed when it is necessary to guarantee that a repeated request to the system will not result in the operation being executed again, especially if it changes the state (for example, creating an order or deducting funds). This is important in distributed systems and in unstable network connections, where a client may repeat a request due to a timeout or error. An idempotent key allows the server to recognize a repeated request and return the same result without executing the operation again, preventing data or action duplication.