1. Make a furnace.
2. Put one of what you want to duplicate in the furnace ingredient.
3. Hit the furnace with a pickaxe until it is about 90% destroyed.
4. Then switch to your hands and destroy the rest of the furnace.
5. It should glitch and reappear, then you have to go into the furnace and hold A on the item in the ingredient spot until the furnace disappears.

You might have to do it a couple of times, but you will have a duplicate item!

PS: to use the items you have to put the items individually into your inventory!