In MZ's notes, it says in RESTful service, "Delete" is defined to be idempotent meaning "multiple identical requests should have the same effect as a single request".
But I think deleting a resource for the first time return status = ok, but deleting the same resource for the second time, it will return status = Not found.
I believe delete is non-idempotent.
It may be counter-intuitive, but DELETE is idempotent. The result of successive request to remove an object are the same - the object is deleted. The status returned to the client may be different, but the effect as far as the resource is concerned is the same.