Denial of Service with LAN worlds if someone logs into a LAN server with the host's username

In Minecraft offline mode, when another person logs into a server with the same username as an already-logged in player, the person who logged in first gets kicked. Since LAN servers run in offline mode, this can be abused.

Steps to reproduce:
1) Open Multiplayer and see the existing LAN games. Note the username of the host.
2) Log into Minecraft in Offline mode using the username of the host of the LAN game
3) Join the LAN game
4) The host will be kicked with a "Logged in from another location" message, and every player will be kicked with the message "Server closed".