Okay so heres how it works:
First make sure you're out of battle, then go to your items menu. Now that we have your placement established change an item to one of the two top positions.
This item must be an item usable in-battle(sorry weapons that can be used as items in-battle are ineligible).
Then go get into a battle...doesnt matter with what, then with any character(I advise using the first...common sense...)go to the item menu make sure the item you want to duplicate is selected(either the top left or the top left will be duplicated bases upon which one is selected)
And press Up + A
An enemy should now be being targeted...
Now you should have one more of the item you selected...
Do not try duplicating items past 99...
Im not sure...but Ive heard tell from others that this can cause your save data to become corrupted...Ive never been stupid enough to try it myself... And if you do...or something happens because of this...please don't come whining to me...Im not going to replace your game
Added by: ashaman_kae_17 Dec 6th 2006, ID#1687