Problem while using same slave to login

Hi...

Drupal version used is: 4.65.
Web server used is: apache2.

I'm using the single sign-on module with a master site built by me, and having a problem to log in through the same slave more then once. The problem does'nt appear while deleting the slave's site cookie and using a different session id each time.
I know that the problem is that the same slave cannot get into the first if statement of the nested if:
" if (empty($_SESSION['singlesignon_prior_sid']) || $_SESSION['singlesignon_prior_sid'] != session_id()) { ".

while trying to log in for the seconed time and dont know why...
The master I've built acts fine and does what's needed and imitates the drupal master exactly the same.

I dont understand why there is the need to check the 2 conditions in the "if statment" insted of checking if the user id is not "0" meaning not logged in.
I'd be glade if some one can help me out solve this, so I can use the logic of hte module writter.
Thank...
Yaniv