This is a duplication glitch found way back in Minecraft 1.4. I thought it might be funny to see if it still works, and it does.

The explaination can be found on my Imgur: http://imgur.com/a/YAgr5

The glitch has been done with an OptiFine installation, but is also doable without.

EDIT: Because there is some confusion, I've recorded myself performing it: https://www.youtube.com/watch?v=ee116NWSCu4 (Didn't havce much time to edit it, sorry)

Have a great day! -MegaXLR