Items can be duplicated using the Survival Inventory tab in the Creative inventory in multiplayer (possibly single player)

This doesn't really matter since you have access to all items but its a bug nonetheless.

To duplicate:
1) Click item to hold it under mouse
2) Double click item in empty spot, putting it down and picking it back up very quickly
3) Very quickly, move mouse to adjacent inventory spot and click. There should be a duplicated item in both spots.

This is exactly the same as a duplication glitch from an alpha version.