In your shop, you can duplicate items. First, take out the item you wish to duplicate and then move your character within your shop (it doesn't matter where). Then, press B to put away the item . Quickly, before the animation depicting your character putting the item away concludes, select a different item from your inventory. Press A to confirm that selection, and if you did it properly you will now have duplicated the item. The process must be completed very quickly or it won't work.