The SEssion Tracking can be Recognized bt the client Browser
whrn use session.getSession() ..rite../ getValue()
The Only Way i see ..for u r problem is
when ever the uaser first logs in
as soon as he/she logs in store the information about the
the user in the data base...
when ever he/.she logs in check the data base ...
when he/she logs out ..delete the record of the user from the database..
this ensures the he/she logs in only one time..
Any Discuusion more on this
Hope u GOT IT..