I found a glitch where you are able to duplicate items when smelting them in a furnace. I was able to recreate this glitch many times, and it is quite easy to do. In order to complete this glitch the player must be holding a pickaxe, and in the furnace UI smelting something. By spamming shift+ and left clicking until the item is done smelting, and exiting the UI right as the item is picked up and put in your inventory, when you open up the furnace again, there is still an item in the furnace that can be picked up and used, as well as the one that was put in your inventory.