Hello all. Me and a few of my friends are trying to play Minecraft Multiplayer using Hamachi (a LAN type service). So here's our problem.

Everytime either one of us tries to connect to the server while another one is on it, it says, "You have logged in from another location." Then signs the player playing out, replacing him with the person joining. We don't have the same IP's either.

I cant seem to figure out what's wrong. Could someone please help?
Oh, and I have the all the Minecraft hosting ports open, (25565) so I don't think thats the problem.