How-to-avoid-multiple-positions-in-C-coded-Rotation-strategy

I am trying to add subsequent coding so that I only have 3 participants and have that working correctly. However I am getting multiple positions and I only want single positions in the same symbol. How do I modify the following code so that there are no duplicates in the Buy List?

CODE:
   public override void PreExecute(DateTime dt, List<BarHistory> participants)
      {
         // rank the candidates by rsi
         foreach (BarHistory bh in participants)
         {
            IndicatorBase symbolRsi = (IndicatorBase)bh.Cache[seriesKey];
            int idx = GetCurrentIndex(bh);  //this returns the index of the BarHistory for the bar currently being processed
            double rsiVal = symbolRsi[idx];

            //if the indicator isn't valid set a high value to avoid selection in rotation
            if (idx < symbolRsi.FirstValidIndex)
               rsiVal = 1.0e6;

            bh.UserData = rsiVal; //save the current Rsi value along with the BarHistory instance
         }

         //sort the participants by Rsi value (lowest to highest)
         participants.Sort((a, b) => a.UserDataAsDouble.CompareTo(b.UserDataAsDouble));

         //keep the top 3 symbols
         buys.Clear();
         for (int n = 0; n < 3; n++)
         {
            if (n >= participants.Count)
               break;
            buys.Add(participants[n]);
         }
      }
      //execute the strategy rules here, this is executed once for each bar in the backtest history