Why ListBox drops two element to another list adds one extra copy using ListBoxDragDropTarget?

I am working on silverlight 5.

ElementDroppingList and ElementReceivingList are my two ListBox and a text is dropped from one to another. The problem is when i drop a text from ElementDroppingList to ElementReceivingList then it always shows 1 extra copy (dont know why). And another thing to note is the one original element (or my be the not original one) go in sorted order and another one show at last of the ListBox.

I have used HashSet even then it do not avoid this extra element.

public void DropFromOneListToAnotherList(ObservableCollection<object> droppedItem) 
    {       
        if (droppedItem != null) 
        {
            HashSet <Function> funcList = new HashSet <Function> ();
            foreach(object func in droppedItem)
            funcList.Add((Function) func);
            foreach(Function func in funcList.OrderBy(c => c.Label)) 
            {               
                ElementReceivingList.Add(func);
                ElementDroppingList.Remove(func);
            }
            SortListBox(); //it sorts in alphabetical order
        }
    }