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.
Various passwords
Go to the bookshelf that is in the corner of your room, go to the "Password" section and choose the mobile symbol.