Use IsGameModeAvail for duet check

Vocaluxe/SongQueue/CSongQueue.cs

@@ -69,7 +69,7 @@ namespace Vocaluxe.SongQueue
 
         private bool _AddSong(int songID, EGameMode gameMode)
         {
-            if (gameMode == EGameMode.TR_GAMEMODE_DUET && !CSongs.GetSong(songID).IsDuet)
+            if (CSongs.GetSong(songID).IsGameModeAvailable(gameMode))
                 return false;
 
             _SongQueue.Add(new SSongQueueEntry(songID, gameMode));