Equip a bow and any set of arrows, then draw your bow back. While it is 
drawn back, go to the
inventory screen. Select the arrows that you are using, then go to the item 
to be duplicated. Drop the
item, then leave the equipment screen and resume the game. The item that you 
dropped will be
duplicated. You can pick up all the duplicates and use or sell them. The 
amount of copies of an
item you make depends on how many arrows you have equipped. For example, 10 
arrows allows
10 copies. This works with most, but not all, items and weapons.
Note: Do not try to duplicate more than 250 copies of an item or the game 
may freeze.