1: First your have to add a server of your choice to your favorites.
2: Second you have to join the server from your favorites and wait until it says "WAITING FOR HOST" in the middle of the screen and "Ready" On the bottom left corner of your screen.
imgur: the simple image sharer <----This screen
When you get to that screen spam the ESC key as fast as you can until you get kicked out to the main menu.
3: Then quickly join back to the same server.
4: When you join the game you need to move backward and you should see a copy of your self.
imgur: the simple image sharer
5: Then proceed to killing your duplicated person.
imgur: the simple image sharer
WARNING: If you kill your dupe with a melee weapon it will damage your duplicated items.
6: Lastly proceed to gathering your duplicated items.
imgur: the simple image sharer
It only seems to work around 1/4th of the time and I think it is because when you do it once your duplicated person will stay on the server you did it on until it restarts so it will only work after that server restarts.
-edit-
Found a video that may better explain this for anyone still confused-
https://www.youtube.com/watch?v=-m72pKsh8qk