Single Player Duplication:

This glitch/cheat requires 2 controllers. First sign into the profile you want to do the duplication on and then start up the world. From here place the items you would like to duplicate in a chest. After this turn off auto save, after it is turned off save. Then grab your second controller and sign into a profile (any one will do). Then what you want to do is go to the chest with the second controller and put it in your inventory. Then save the game with the second controller, not the first this time. After you’ve saved put the items back in the chest and sign out the second controller. Sign back in and you should have all the same items in your inventory. Place them in the chest. You should have double the items in the chest now. Repeat as many times as desired.