https://issues.apache.org/jira/browse/AMQ-5066

apply patch after review

activemq-mqtt/src/main/java/org/apache/activemq/transport/mqtt/MQTTProtocolConverter.java

@@ -366,7 +366,17 @@ public class MQTTProtocolConverter {
     }
 
     QoS onSubscribe(Topic topic) throws MQTTProtocolException {
-        if( !mqttSubscriptionByTopic.containsKey(topic.name()) ) {
+
+        if( mqttSubscriptionByTopic.containsKey(topic.name())) {
+            if (topic.qos() != mqttSubscriptionByTopic.get(topic.name()).qos()) {
+                // remove old subscription as the QoS has changed
+                onUnSubscribe(topic.name());
+            } else {
+                // duplicate SUBSCRIBE packet, nothing to do
+                return topic.qos();
+            }
+        }
+
         ActiveMQDestination destination = new ActiveMQTopic(convertMQTTToActiveMQ(topic.name().toString()));
 
         ConsumerId id = new ConsumerId(sessionId, consumerIdGenerator.getNextSequenceId());
@@ -383,7 +393,6 @@ public class MQTTProtocolConverter {
         mqttSubscriptionByTopic.put(topic.name(), mqttSubscription);
 
         sendToActiveMQ(consumerInfo, null);
-        }
         return topic.qos();
     }
 

activemq-mqtt/src/main/java/org/apache/activemq/transport/mqtt/MQTTSubscription.java

@@ -67,4 +67,8 @@ class MQTTSubscription {
     public ConsumerInfo getConsumerInfo() {
         return consumerInfo;
     }
+
+    public QoS qos() {
+        return qos;
+    }
 }

activemq-mqtt/src/test/java/org/apache/activemq/transport/mqtt/MQTTTest.java

@@ -377,6 +377,58 @@ public class MQTTTest extends AbstractMQTTTest {
 
     }
 
+    @Test(timeout = 60 * 1000)
+    public void testDuplicateSubscriptions() throws Exception {
+        addMQTTConnector();
+        brokerService.start();
+
+        MQTT mqtt = createMQTTConnection();
+        mqtt.setClientId("foo");
+        mqtt.setKeepAlive((short)2);
+
+        final int[] actualQoS = {-1};
+        mqtt.setTracer(new Tracer() {
+            @Override
+            public void onReceive(MQTTFrame frame) {
+                // validate the QoS
+                if (frame.messageType() == PUBLISH.TYPE) {
+                    PUBLISH publish = new PUBLISH();
+                    try {
+                        publish.decode(frame);
+                    } catch (ProtocolException e) {
+                        fail("Failed decoding " + e.getMessage());
+                    }
+                    actualQoS[0] = publish.qos().ordinal();
+                }
+            }
+        });
+
+        final BlockingConnection connection = mqtt.blockingConnection();
+        connection.connect();
+
+        final String RETAIN = "RETAIN";
+        connection.publish("TopicA", RETAIN.getBytes(), QoS.EXACTLY_ONCE, true);
+
+        QoS[] qoss = { QoS.AT_MOST_ONCE, QoS.AT_MOST_ONCE, QoS.AT_LEAST_ONCE, QoS.EXACTLY_ONCE };
+        for (QoS qos : qoss) {
+            connection.subscribe(new Topic[]{ new Topic("TopicA", qos) });
+
+            final Message msg = connection.receive(5000, TimeUnit.MILLISECONDS);
+            assertNotNull(msg);
+            assertEquals(RETAIN, new String(msg.getPayload()));
+            int waitCount = 0;
+            while (actualQoS[0] == -1 && waitCount < 10) {
+                Thread.sleep(1000);
+                waitCount++;
+            }
+            assertEquals(qos.ordinal(), actualQoS[0]);
+        }
+
+        connection.unsubscribe(new String[]{"TopicA"});
+        connection.disconnect();
+
+    }
+
     @Test(timeout=60 * 1000)
     public void testSendMQTTReceiveJMS() throws Exception {
         addMQTTConnector();