Trying to avoid duplicate attribute getting by following code.

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C#3.5

AutoCAD

[CommandMethod("NLTAG")]
             public void ListAttributes()

             {
            Document acDoc = Application.DocumentManager.MdiActiveDocument;
            Editor ed=Application.DocumentManager.MdiActiveDocument.Editor;
            Database db = HostApplicationServices.WorkingDatabase;
            Transaction tr = db.TransactionManager.StartTransaction();

            try
            {


            TypedValue[] filList = new TypedValue[1] { new                           TypedValue((int)DxfCode.Start, "INSERT") };
           SelectionFilter filter = new SelectionFilter(filList);

           // Do nothing if selection is unsuccessful
           if (res.Status != PromptStatus.OK)
               return;

           SelectionSet selSet = res.Value;
           ObjectId[] idArray = selSet.GetObjectIds();



           foreach (ObjectId blkId in idArray)
           {
               BlockReference blkRef = (BlockReference)tr.GetObject(blkId, OpenMode.ForRead);
               BlockTableRecord btr = (BlockTableRecord)tr.GetObject(blkRef.BlockTableRecord, OpenMode.ForRead);
               ////ed.WriteMessage(
               ////  "\nBlock: " + btr.Name
               ////);
               btr.Dispose();

               AttributeCollection attCol = blkRef.AttributeCollection;
               AttributeCollection parts = blkRef.AttributeCollection;
               foreach (ObjectId attId in attCol)
               {

               AttributeReference attRef(AttributeReference)tr.GetObject(attId, OpenMode.ForWrite);



     string str = ("\n " + attRef.TextString);//here i get duplicate value
                   ed.WriteMessage(str);
               }
           }
           tr.Commit();
       }
       catch (Autodesk.AutoCAD.Runtime.Exception ex)
       {
           ed.WriteMessage(("Exception: " + ex.Message));
       }
       finally
       {
           tr.Dispose();
       }

What I have tried:

This is my c# code which get all attributes from selected block and print it and i want to avoid the duplicate attributes so i get each attribute onec only.