To duplicate an item, first save the game. Trade the item to be duplicated to another player for no item. Exit to the main menu. Answer "No" when prompted to save the game. Reload the previously saved game. You will still have the previously traded item. Have the other player save their game, have the item traded back to you. Save the game. Have the other player exit to the main menu and answer "No" when prompted to save. Both players should now have the same item.