This method only works on equipment that you can wear; it does not work on consumable items and the like. Put the item you want to dupe in your Item Bag, then fill both your Item Bag and Warehouse up to the max, so you can't buy anymore items. Now, go to the Item World and choose the item you want to dupe as the item to go into. While you're in the Item World, you need to pick up an item, either from a chest or from the Bonus Gauge. Now, you must make it to floor 10 and use that to exit the Item World, as using a Mr. Gency Exit would use an item and leave an empty slot. When you do exit the Item World, you'll notice the NPC say that the item you're trying to dupe cannot be place in your inventory and is now in the store, so make some room in your inventory now. Go to the store (weapon or armor) and TRY ON the piece of equipment with Square, then use the option to buy it while trying it on. If you buy it outright, it will remove the equipment from the store. If you did it right, you should have a copy of the item in your inventory and the item should still be in the store. Keep doing until to your heart's desire. Note that if you save the game and reload it, the item will be gone from the shop.