trikky authentication question

hi guys i have an authentication class , there is one last big issue to fix , i am trying to prevent multiple logins , it does this ok except , the first login gets kicked instead of the second one , i have a last_login date entry to work with , what else should i have so on the login check if the user is logged in , there is an issue using the logged in feature , ok u give it an interval of say 2 hours , this may prevent that person reloggin in for two hours right ?? :| , or say its a few minutes , i can log bak in after a few minutes and still kick the first login, what are my options