prevents login with same username at the same time

[eluser]dpsamarasekara[/eluser]
hi,

i'm doing a site and i need to prevents login with same username at the same time.but there are several other modules bind to main system and they have own codeigniter installations. but loggin is centralized.so same user can't log in two places using his username. if so i need to give msg "username is in use" .

how can i do this.