Instructions
Go to your crafting table and build a furnace. You need eight pieces of cobblestone to craft one furnace.
Put the item you want to duplicate in the furnace.
Jump on top of the furnace and equip your pickaxe. Strike the furnace repeatedly with the pickaxe.
Equip your shovel right before the furnace falls apart. The furnace will break and then reappear.
Open the furnace. Highlight the item you want to duplicate and press "A" rapidly until the furnace breaks again. If you do this correctly, you'll see two items on the ground when the furnace breaks.
Pick up one of the items and then check your inventory. If you're holding the duplicated item, you'll see 64 copies of the item in your inventory.
Open your chest. Press and hold "X" to transfer the batch of duplicated items into the chest.