It doesn't seem possible. Are you sure nobody else is visiting the site? The insert gets the value from the recordset and the recordset gets the value from the url. The multiple clientID records could only be inserted if someone visited that url with the associated clientID values.
Whatever the issue is, I'm pretty confident it isn't with the page itself.
Maybe try removing the .js files the page is using to see if they are somehow causing the page refresh.
Another option might be to add a few more fields to the database for debugging. Save the value for <?php echo($_SERVER['REQUEST_URI']); ?>
and the value of <?php echo(__FILE__); ?> and the value of <?php echo(session_id()); ?> during the insert. That would tell you the url of the page when it inserts, the page name doing the insert, and the sesison id of the inserting user.
Maybe that would shed light on it. Something strange is happening, but it is really hard to determine why or how that clientID would change or even could change.