sing proxy causes problems in a PHP application writing in a MySql database ?

[coche]

My PHP-Mysql application is dealing with satisfaction surveys, it is working well for many users but has problems with a customer who uses Proxy+ to share his permanent internet connexion between 5 users.
The program uses session and registers some variables;

when using the Proxy , the data in 2 files are registered 2 times (same ) with a, INSERT..
I am not sure it is the proxy... because sometimes with big data, it is ok
It does the same as if we press the "refresh" button..

some code:
global $PHPSESSID;
if ($PHPSESSID)
session_start($PHPSESSID);
else
session_start();

session_register("user");
session_register("password");
session_register("base");

and further....

in saisie_enq.php3
print "<form method=\"post\" action=\"savenewenq.php3\">";
print ('Date de réponse des enquêtes (jj/mm/aaaa) ');
print ('<input type=text size=2 maxlength=2 name="jour"><input type=text size=2 maxlength=2 name="moisdate"><input type=text size=4 maxlength=4 name="annee"><font size="1"> (obligatoire)');
.....with the fields...
print ("<BR><BR><INPUT TYPE=\"Submit\" VALUE=\"OK\"><input type='reset' value='Annuler'>\n");
print ("</FORM>\n");

and then in savenewenq.php3
$query = "INSERT INTO saisie (no_saisie,date_saisie,user,code_questionnaire,service)"
."VALUES('NULL', '$datesaisie', '$user', '$questionnaire', '$service')";

if (!mysql_query ($query))
{
print ("Envoyer email sur [e-mail] pour signaler l'erreur ");
print ("<br>");
print ("Numéro d'erreur sur saisie : " . mysql_errno());
print (" erreur: " . mysql_error());
print ("<br>Utiliser le lien ci-dessous pour revenir à la saisie
<br> <a href='saisie_enq.php3'>Retour saisie</a>");
die(erreur_mysql("Erreur insert saisie",$query));

        }

/

$dernier_no_saisie = mysql_insert_id();

$i = 2;

while ($question[$i] != '')

{

$query = "INSERT INTO reponse (no_saisie,code_quest,answer) VALUES('$dernier_no_saisie','$question[$i]', '$Reponse[$i]')";

   if (!mysql_query ($query))
   {
        print ("Envoyer email sur [email][e-mail][/email] pour signaler l'erreur ");
        print ("<br>");
        print ("Numéro d'erreur sur reponse : " . mysql_errno());
        print (" erreur: " . mysql_error());
        print ("<br>Utiliser le lien ci-dessous pour revenir à la saisie
        <br>  <a href='saisie_enq.php3'>Retour saisie</a>");
        die(erreur_mysql("Erreur insert reponses",$query));
   }

}
$i++;

MAJ_BASE

print ('<META HTTP-EQUIV="refresh" content="2; URL=saisie_enq.php3?PHPSESSID='.$PHPSESSID.'">');

Help Please