user already logged in [RESOLVED]

i am pondering now how to set up my application so that if a user is logged in then they cannot login again

i was thinking of putting a field in the db (logged in)
1 if they are logged in
0 if they are not

then

if logged in = 1 where username
then give error
"youre already logged in so dont even try it"

would this be the way to do it.. ;)

THIS THREAD HAS BEEN PREVIOUSLY ANSWERED BY MENDHAK

this seems to be they way to do it.
thanks mendhak for your reply, just found it there now, sorry
moderator feel free to delete this thread