Step 1: Put item in your inventory.
Step 2: Drop item (More items dropped = more items duplicated)
Step 3: Pause and click "Save and Quit To Title"
Step 4: Play the world with dropped items
Step 5: Pick up items.
Step 6: Pause. Wait of the world to save.
Step 7: Exit Minecraft.
Step 8: Open Minecraft again.
Step 9: Enjoy the duplicated items! (Works with diamonds) tooken from ign