Growlanser 4: Over Reloaded
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This game is titled Growlanser: Wayfarer Of Time in North America and Growlanser 4: Over Reloaded in Japan.
Have only one copy of the Spellstone you want to duplicate equipped and make sure you have none in your inventory. Get in a random battle. Unequip the spellstone that you want to duplicate, then have Crevanille target someone. After selecting "Confirm" on the enemy target, press Triangle to bring up the menu. Equip the spellstone that you just unequipped from Crevanille to another character, then complete the battle.
Strategy guides from GameFAQs