To add to what Mark says, Kerberos doesn't help prevent multiple logins.
That isn't what it is intended to do. In fact, Windows Server 2003
doesn't have any built-in way of preventing multiple concurrent logins
from a single username and it uses Kerberos in a domain environment for
doing the authentication so that tells you right there kerberos isn't
the answer.

You will need to have a central repository for keeping track of who is
logged in and testing against that data repo everytime someone logs in
to see if they are already logged in. Again, in the case of Windows
there isn't anything that already exists that can do that and  I've used
a new attribute within the Active Directory schema to keep track of it.
You will have to do something like that on the server you are working
with (some flag or something) in order to prevent multiple concurrent
logins.