[FLINK-3981] don't log duplicate TaskManager registrations

Duplicate TaskManager registrations shouldn't be logged with Exceptions
in the ResourceManager. Duplicate registrations can happen if the
TaskManager sends out registration messages too fast when the actual
reply is not lost but still in transit.

The ResourceManager should simply acknowledge the duplicate
registrations, leaving it up to the JobManager to decide how to treat
the duplicate registrations (currently it will send an AlreadyRegistered
to the TaskManager).

This closes #2045

flink-runtime/src/main/java/org/apache/flink/runtime/clusterframework/FlinkResourceManager.java

@@ -354,18 +354,19 @@ private void handleRegisterResource(ActorRef jobManager, ActorRef taskManager,
 		ResourceID resourceID = msg.resourceId();
 		try {
 			Preconditions.checkNotNull(resourceID);
-			WorkerType newWorker = workerRegistered(resourceID);
-			WorkerType oldWorker = registeredWorkers.put(resourceID, newWorker);
+			// check if resourceID is already registered (TaskManager may send duplicate register messages)
+			WorkerType oldWorker = registeredWorkers.get(resourceID);
 			if (oldWorker != null) {
-				LOG.warn("TaskManager {} had been registered before.", resourceID);
+				LOG.debug("TaskManager {} had been registered before.", resourceID);
 			} else {
+				WorkerType newWorker = workerRegistered(resourceID);
+				registeredWorkers.put(resourceID, newWorker);
 				LOG.info("TaskManager {} has registered.", resourceID);
 			}
 			jobManager.tell(decorateMessage(
 				new RegisterResourceSuccessful(taskManager, msg)),
 				self());
 		} catch (Exception e) {
-			// This may happen on duplicate task manager registration message to the job manager
 			LOG.warn("TaskManager resource registration failed for {}", resourceID, e);
 
 			// tell the JobManager about the failure