Duplicating is easy in Minecraft, but you need a friend to help you out. You can then make up to 64 on an item.

1. Put the item you watch to dupe in a Dispender.

2. Have you mate break the Dispender.

3. You take then item, which will now be 64 items! Put it in your inventory.

4. From there, put the item in the chest and exit.

5. Then, go back to the chest and one item at a time, place them.

It works!