Minecraft Multiplayer Registering Problems? Quick?

Every time I go on a server, I have to login or register. The way to register is /register (password) (ConfirmPassword) so I do that and then it says "Username already registered". I have never been on this server before. I then try to login and it says invalid, and disconnects me. I haven't bought the game, but I get on SOME servers fine. (Some say i should try restarting my game?) Someone help me!