Session logic question

I use the $_SESSION buffer extensively on my site without a problem, except one. Maybe someone can help me understand the problem. I have a simple page counter with a test to see if the page has been counted for the user. start_session(); * * [additional code] *

   if($_SESSION['ctr_file']!==$counterFile)             //See if visitor has already been counted for this page
   {$num = $num+1;        $_SESSION['ctr_file'] = $counterFile;                //set/reset counter file for next time

Pages that use the counter function all have a start_session(); and a call to the counter function, Everything works fine, except when I include another script file that also uses the $_SESSION buffer. What appears to be happening is that start_session() on the second script reinitializes the session buffer and I lose the data from the first session. Yet, as I read php manual, it seems to say that start_session is suppose to simply restore the variables for the page it is on; not start a new session. many thanks....