If you're doing well-formed RPC over POST, as opposed to ad hoc RPC (which, let's be honest, is the accurate description for many "REST" APIs in the wild), then requests and responses should have something like an `id` field, e.g. in JSON-RPC:

https://www.jsonrpc.org/specification#request_object

Commonly, servers shouldn't accept duplicate request IDs outside of unambiguous do-over conditions. The details will be in the implementations of server and client, as they should be, i.e. not in the specification of the RPC protocol.