Note: Save your progress and make a backup of your save before attempting this; alter game files at your own risk. Deposit the item you want in a chest, then save the game and exit. Go back in the game,withdraw the item you want to clone, and put it in your inventory. Wait 5 min (very important), then close the game via Alt-F4. Go back into and there is your cloned item. You should now have the same item in the chest and in your inventory.