Fixed timing issue allowing multiple instances of the same command to be queued

src/NzbDrone.Core/Messaging/Commands/CommandQueueManager.cs

@@ -96,8 +96,10 @@ namespace NzbDrone.Core.Messaging.Commands
             _logger.Trace("Publishing {0}", command.Name);
             _logger.Trace("Checking if command is queued or started: {0}", command.Name);
 
+            lock (_commandQueue)
+            {
                 var existingCommands = QueuedOrStarted(command.Name);
-            var existing = existingCommands.SingleOrDefault(c => CommandEqualityComparer.Instance.Equals(c.Body, command));
+                var existing = existingCommands.FirstOrDefault(c => CommandEqualityComparer.Instance.Equals(c.Body, command));
 
                 if (existing != null)
                 {
@@ -123,6 +125,7 @@ namespace NzbDrone.Core.Messaging.Commands
 
                 return commandModel;
             }
+        }
 
         public CommandModel Push(string commandName, DateTime? lastExecutionTime, CommandPriority priority = CommandPriority.Normal, CommandTrigger trigger = CommandTrigger.Unspecified)
         {