Remember to SAVE your progress and make a backup of your save. Alter game files at your own risk. Deposit the item you want in a chest. Save the game and exit. Go back in the game and withdraw the item you want to clone and put it in your inventory. Wait 5 min (very important). 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.