What do you mean it has nothing to do with REST? It's an application built on HTTP.
The same issues do not exist. Response 2 does not return "silent success" it returns 409 Conflict. The client can distinguish between conflict & success.
The error codes nor the transport are important here.
You can make an arbitrary number of requests that can respond with various responses (or fail) in arbitrary order. Who do you listen to? Which response is 'correct'? Sure you can build a service on the client to track the requests, the order they are issued in, what responses supersede other responses, how long to wait for all responses, etc, etc. But why build such a complicated solution to such a simple problem?
