Follow these steps
1: Save the game, preferably under quicksave.
2: Drag the item out of your inventory onto a place it can't go (so a big red X appears on it)
3: Whilst still holding it down, load the game using quickload.
4: When it is loaded, you should have duplicated the item you dragged, both should be in your inventory, or 1 in your inventory and one near the place you dragged it.
5: This can be done with more than one item, e.g.: Drag 2 potions and do the duplication trick and you will have 4.