Dropping an Item and closing the World at the same time as you pickup the Item will let you duplicate the item.
How to reproduce:
1. Drop an item on the Ground below you
2. Wait until you pick the item up again
3. Hit Esc at exactly that moment (you should see the pickup animation of the Item in your Hotbar and still the Item on the Ground)
4. Exit the World
4.5 Some people are saying that you have to quit minecraft and relaunch it for the Bug to "work"
5. Reload the World
6. See that you have 2 Items in your Inventory
Here is a german Video showing the Bug (without relaunching Minecraft): https://www.youtube.com/watch?v=wvyUARstSCo