Fix MQTT v5 overlapping subscription behaviour.

Clients now receive message from all matching subscriptions rather than
the first one encountered, which ensures the maximum QoS requirement is
met.

ChangeLog.txt

@@ -25,6 +25,9 @@ Broker:
 - Fix incorrect shared subscription topic '$shared'.
 - Fix zero length client ids being rejected for MQTT v5 clients with clean
   start set to true.
+- Fix MQTT v5 overlapping subscription behaviour. Clients now receive message
+  from all matching subscriptions rather than the first one encountered, which
+  ensures the maximum QoS requirement is met.
 
 Client library:
 - Fix typo causing build error on Windows when building without TLS support.

src/database.c

@@ -364,7 +364,8 @@ int db__message_insert(struct mosquitto_db *db, struct mosquitto *context, uint1
 	 * multiple times for overlapping subscriptions, although this is only the
 	 * case for SUBSCRIPTION with multiple subs in so is a minor concern.
 	 */
-	if(db->config->allow_duplicate_messages == false
+	if(context->protocol != mosq_p_mqtt5
+			&& db->config->allow_duplicate_messages == false
 			&& dir == mosq_md_out && retain == false && stored->dest_ids){
 
 		for(i=0; i<stored->dest_id_count; i++){