Start by placing any items you plan to duplicate in your inventory. Now pause the game and tap "Back Up," after which you should move the desired items from your inventory and into a chest. Now exit the game and head to the Options page. Click the green button to restore items, and when you return to the game you will have the items that were in your inventory. The items that you had put in the chest will still be there, though, effectively doubling your supply. You can repeat the cheat as often as you like.