Fix for MQTT-3.8.4-3.

src/subs.c

@@ -229,6 +229,19 @@ static int _sub_add(struct mosquitto_db *db, struct mosquitto *context, int qos,
 			leaf = subhier->subs;
 			last_leaf = NULL;
 			while(leaf){
+				if(leaf->context && leaf->context->id && !strcmp(leaf->context->id, context->id)){
+					/* Client making a second subscription to same topic. Only
+					 * need to update QoS. Return -1 to indicate this to the
+					 * calling function. */
+					leaf->qos = qos;
+					if(context->protocol == mosq_p_mqtt31){
+						return -1;
+					}else{
+						/* mqttv311 requires retained messages are resent on
+						 * resubscribe. */
+						return 0;
+					}
+				}
 				last_leaf = leaf;
 				leaf = leaf->next;
 			}