When trading, you can get ghost items to appear in private storage. I've verified that the item doesn't actually duplicate, but it is very confusing.
steps to duplicate:
1) player A: open a trade window to player B
2) player A: drag item X to it to trade -> send offer
3) player B: drag another item to player B's trade window
4) player B: ghost item X shows up in player B's private storage
ghost item X will disappear on its own if you move it to another storage area. if you don't do that, you can get info on it or do something really wacky like put ghost item X in the trade window. however, the trade window will correctly note that the item doesn't actually exist when you click on send offer.