Quote Originally Posted by Timmie
You must first have at least a quantify of two (2) of the item you want to duplicate.

Proceed to get into a battle and when it's the turn of the party member with the W-Item materia equipped, select the W-Item command from the menu. Select the item to be duplicated, then select a friend or enemy on whom to use the item (doesn't matter, because you don't have to use it). Then, select the same item (a second time), and the hand will come up giving you the option to select a target. Cancel with the X button (default), then select the item again so that you're prompted to select a target.

Cancel again. Select the item, cancel, select, cancel, select, cancel, and repeat. Each time you do this, the item should increase in stock by one.
Yes, although you only need one item in order for it to be duplicated. Simply use the desired item, and then follow the select/cancel process with a different item. The result is the same, although it makes it much easier to duplicate items like Cauldrons and Vagyrisk Claws.