Item Duplication
Unverified. Item infinite duplication (this is for the PS3 version). Save before doing this glitch, just in case.

Get as many scrolls as you want of the same type.
Click twice on the scrolls.

Drop the item you want duplicated. It has to have a quantity of one. So drop the extra quantity first.
Exit the menu and collect your items.

If you have 10 scrolls of the same type, you'll get 10 items. If you have 70 scrolls of the same type, you'll get 70 items. Be careful. If you duplicate too many items at the same time the game can freeze.

Example: with lockpicks it is better to duplicate in small quantities, about 20 at a time is the limit. I duplicated more and I had to restart the system.

Cheat - Duplicate Items (Post Patch)
This cheat duplicates items in a similar way to the pre-patch method using a bow and arrows. I have seen variations on this method involving swinging weapons/fists but found they did not work (on the PC at least). This method works with Shivering Isles installed and is more straight forward than the old method.

To start with you will need at least 2 identical scrolls. Open you inventory and equipment them. Click on the scrolls once more, then drop the item you would like to duplicate. The quantity of the item needs to be less than the number of scrolls, for example if you have 10 scrolls the number of items duplicated at one time has to be 9 or less. Exit your inventory screen and the duplicates will fall to the ground. The number of items duplicated will be equal to the number of scrolls. To repeat this go back into the inventory screen and click on the equipped scrolls and drop the item to be duplicated (no need to unequip and equip the scrolls)

This method does not duplicate stolen items and some rare/quest items. It is however useful for duplicating light, valuable items such as gemstones which can be sold.