After some weird experiences (and some messing around to discover the probable cause) I discovered that if you lose connection to the server via unplugging your internet cable (or whatever means necessary to lose your internet connection) it doesn't log you off the server. This causes some weird mishaps, as well as some duplication possibilities. (One time I lost connection while surrounded by zombies, and when I logged back on I was alive, but copies of my inventory was all over the ground. No screen-caps of that though)

What I expected to happen was...:
When I lost my internet connection I would leave the server.

What actually happened was...:
I remained on the server, and when I logged back on there were more of me.

Steps to Reproduce:
1. Join a server over the internet
2. Unplug your internet cable
3. You should get the message: "Internal Exception: java.io.IOException: An existing connection was forcibly closed by the remote host"
4. Plug your internet back in and re-connect to the server (it appears that this has to happen relatively soon, otherwise you'll become the "original" you like expected)
5. There are now two of you. If you kill the "original" and then re-log a while later you will become the "original" (and of course be dead)