Add motivation for duplicate push ids

draft-ietf-quic-http.md

@@ -1240,6 +1240,13 @@ and detects a mismatch, it MUST respond with a connection error of type
 H3_GENERAL_PROTOCOL_ERROR. If the decompressed header sets match exactly, the
 client MUST ignore the duplicate PUSH_PROMISE frame.
 
+Allowing duplicate references to the same Push ID is primarily to reduce
+duplication caused by concurrent requests.  A server SHOULD avoid reusing a Push
+ID over a long period.  Clients are likely to consume server push responses and
+not retain them for reuse over time.  Clients that see a PUSH_PROMISE that uses
+a Push ID that they have since consumed and discarded are forced to ignore the
+PUSH_PROMISE.
+
 If a PUSH_PROMISE frame is received on the control stream, the client MUST
 respond with a connection error ({{errors}}) of type H3_FRAME_UNEXPECTED.