You'll need these before you go on with this trick (but I didn't use this trick in my game because I don't have a the materia needed.)
a. The W-Item materia
b. At least 2 of the item you want to duplicate (for example, a Megalixir, then you must have at least 2 Megalixirs.)
First, you have to equip the W-Item materia with one of your characters, and then fight a monster. In the middle of the fight use the character you have the W-Item materia equipped with and select W-Item from the menu, and then select the item you want to duplicate. Press circle and then pick who you want to use it on, and then press circle again. Now you will be asked again which item you want to use. Select the item you selected previously then press circle, and when the computer asks you who to use it on, press X to cancel and you'll get your item back with an extra one! Keep pressing O and X rapidly to get as many of the items as you want (maximum is 99). Use this code to get more ELIXIRS to give to the magic pots in the Northern crater. Give them an elixir then kill them and you'll get a lot of EXP and AP use this tactic to build up.