added duet filter

PartyModeChallenge/PartyModeChallenge.cs

@@ -149,6 +149,7 @@ namespace Vocaluxe.PartyModes
 
             _ScreenSongOptions.Sorting.SearchString = String.Empty;
             _ScreenSongOptions.Sorting.SearchStringVisible = false;
+            _ScreenSongOptions.Sorting.ShowDuetSongs = false;
             
             _Stage = EStage.NotStarted;