Because only the caller knows if two payloads, despite being identical, are genuinely different requests or retries of the same request. That said, in this age

Because only the caller knows if two payloads, despite being identical, are genuinely different requests or retries of the same request. That said, in this age of micro services and large-scale distributed systems a request may get retried by a framework layer down a call-chain regardless of whether the request originator intended it or not.