You can fill up your session dir and/or have issues with cookies. Remember to destroy your session somewhere, too, and setup a cleanup crontab.
That's ugly, but it'll likely do what you want.
Code:
@start_session();
$_SESSION['clicks'] = (isset($_SESSION['clicks'])) ? (int)$_SESSION['clicks']+1 : 1;
if ($_SESSION['clicks'] > 1){
header ("location: join form");
}