This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
You can use the W-Item materia to duplicate any equippable items that
your party may have.
Give a character the W-Item materia. Then, get into a battle and unequip
the W-Itemed character's weapon and equip something else. Your
unequipped weapon should have duplicated itself.