In the original NES version of Final Fantasy III, it is possible to duplicate magic using the Sight spell. To use this glitch, one must be on the world map on foot or on airship, to be able to use Sight.
One must go into the magic menu and remove all the spells one wants duplicated, and not leave the menu.
One must use the Sight spell, and all the spells removed will still be there, yet they will have appeared in the player's inventory.
This glitch can effectively be used to generate money, by selling duplicates of spells. It is especially useful later in the game, as Shurikens are extremely expensive.