If you've downloaded the latest official patch for the Shivering Isles, there is still a way to duplicate most stackable items. This method involves scrolls rather than arrows.
First, you’ll need multiples of a single scroll. You might be able find more than one of a scroll for sale in one of the magic shops in the major cities. Once you have that, take out a melee weapon and swing. Open up your character’s inventory in the middle of the swing, select the stacked scrolls, and click on them twice. Then, look for the item you wish to duplicate and drop it. If done correctly, that one item you dropped will turn to the same amount as the stacked scrolls you clicked on.
This glitch only works with stackable items such as potions, gems, and so on.
(works at the time of writing... May be patched to fix in later versions) This video is from our unofficial game guide. You can view the full guide here