How to implement an idempotent POST so that it is not semantically identical to PUT?

Idempotency of POST can be implemented by using a unique request identifier (for example, Idempotency-Key), which the client generates and sends along with the request. The server stores this key and the result of processing the request. If a subsequent request with the same key arrives, the server returns the same response without creating a new resource.

This differs from PUT, which semantically updates or creates a resource at a known URI, whereas POST typically creates a resource with a server-generated identifier.

Example:

• The client sends a POST with Idempotency-Key: 12345 to create an order.
• The server creates the order and saves the result with the key 12345.
• On a subsequent request with the same key, the server returns the already created order without creating a new one.

Thus, POST remains semantically a resource creation, but becomes idempotent through the key, and does not turn into PUT, which updates a resource at a known URI.