The issue in this particular scenario is probably caused by the mentioned PreviewDrop event handler of the RichTextBox. If you have handled the event (by setting the e.Handled = true) the DragDropCompleted event of the ListBox won't be fired and the item won't be added again while reordering. Please try to set the e.Handled to false and let me know if this helps. If it doesn't I'll ask you to share a sample project so we can investigate the scenario on our side.
I'm looking forward to hearing from you.
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