As a server owner I know how stressing it can be when you have people duplicating items. It ruins the economy (if you have one) and everyone has loads of the best items.

So to patch this dupe is very simple. I informed md_5, the developer of spigot of the dupe, and he patched it. Now, if you don't know what spigot is, it's a modified version of craftbukkit that is more leightweight, and has many other benefits. All your plugins and everything will work the same way. You can read more here.

All you need to do is download the latest development build of spigot here, which contains the patch. Replace this with your current server jar (craftbukkit, vanilla, whatever) and start the server back up and the dupe is successfully patched!

Edit: He apparently hasn't patched the nether portal method (second method in the video) yet. The first one is patched though.