-Save the game, then remove an item from your inventory and hold it with the pointer. To hold an item with the pointer, LEFT CLICK it but do not drag it. While the item is held with the pointer, press ESC and select "Restore". Choose to restore the saved game that was just created. You will be prompted to drop the item on the ground. Answer "No". You can now display the inventory screen and place the item back there. You will now have two of that item in your inventory.
-You can also duplicate items from random chests. Press F2 to quick save the game when you find a chest. Take the item if desired after the chest is opened. Enter the inventory screen and hold the recently obtained item with the pointer. Press [Esc] and select "Restore". Select to restore your quick save game. Answer "No" when asked to drop the item, then place it in your inventory. You can now open the chest again and repeat looting the chest.