Ensure unicity within Game.turns

Sources/Model/Games/Game.cs

@@ -96,8 +96,11 @@ namespace Model.Games
                 player,
                 ThrowAll()
                 );
+            if(turn != null && !(turns.Contains(turn)))
+            {
                 turns.Add(turn);
             }
+        }
 
         /// <summary>
         /// finds and returns the player whose turn it is