Drop any item on the ground and walk back about five spaces. Make sure you have
room in your inventory for two of the items. Click on the item and right when
you are picking it up click on any of the items in your belt (the belt is the
area that has 8 spaces for potions and scrolls.) If you did it right the box
where it names the thing your clicker is on should name the thing you are trying
to duplicate. Press the hotkey for inventory (I) and put it in your inventory.
The original item should already be in your inventory. Make sure your inventory
is closed while duplicating until you put the item in.
Thanks to Revolution readers Squin and Bantam!!