added duet filter

VocaluxeLib/PartyModes/CPartyModeNone.cs

@@ -19,6 +19,7 @@ namespace Vocaluxe.PartyModes
 
             _ScreenSongOptions.Sorting.SearchString = String.Empty;
             _ScreenSongOptions.Sorting.SearchStringVisible = false;
+            _ScreenSongOptions.Sorting.ShowDuetSongs = true;
         }
 
         public override ScreenSongOptions GetScreenSongOptions()

VocaluxeLib/PartyModes/IPartyMode.cs

@@ -20,6 +20,7 @@ namespace Vocaluxe.PartyModes
         public EOffOn IgnoreArticles;
         public string SearchString;
         public bool SearchStringVisible;
+        public bool ShowDuetSongs;
     }
 
     /// <summary>