Запрет повторного нажатия на ссылку активации в e-mail

<?
include 'db.php';
$db_scieTitle = $_REQUEST['scieTitle'];
$db_fName = $_REQUEST['fName'];
$db_lName = $_REQUEST['lName'];
$db_regDate = date("Y-m-d H:i:s");
$db_mobPh = $_REQUEST['mobPh'];
$db_eMail = $_REQUEST['eMail'];
$db_orgName = $_REQUEST['orgName'];
$db_depName = $_REQUEST['depName'];
$db_posName = $_REQUEST['posName'];
$db_orgPh = $_REQUEST['orgPh'];
$db_orgFax = $_REQUEST['orgFax'];
$db_orgSite = $_REQUEST['orgSite'];
$db_orgStreet = $_REQUEST['orgStreet'];
$db_orgCity = $_REQUEST['orgCity'];
$db_orgCode = $_REQUEST['orgCode'];
$db_orgCountry = $_REQUEST['orgCountry'];
$db_cmmnts = $_REQUEST['cmmnts'];
 
$result = mysql_query("SELECT id FROM participants WHERE email='$db_eMail'");
$myrow = mysql_fetch_array($result);
if (!empty($myrow['id'])) {
exit ('<p class="rd btxt cntr">Sorry!<br/>The&nbsp;e-mail that you fill in&nbsp;the&nbsp;form already exists in&nbsp;the&nbsp;database.<br/>Please, choose another e-mail for&nbsp;registration!</p>');
}
$insert_sql = "INSERT INTO participants (reg_date, scie_title, first_name, last_name, mob_phone, email, organization, department, position, phone, fax, w_site, adress, city, zip_code, country, comment)" . "VALUES('{$db_regDate}', '{$db_scieTitle}', '{$db_fName}', '{$db_lName}', '{$db_mobPh}', '{$db_eMail}', '{$db_orgName}', '{$db_depName}', '{$db_posName}', '{$db_orgPh}', '{$db_orgFax}', '{$db_orgSite}', '{$db_orgStreet}', '{$db_orgCity}', '{$db_orgCode}', '{$db_orgCountry}', '{$db_cmmnts}');";
mysql_query($insert_sql);
echo '<p class="rd btxt cntr">You have successfully completed the&nbsp;form! Check your email! We have sent you an&nbsp;activation link!</p>';
 
$activ = mysql_query("SELECT id FROM participants WHERE email='$db_eMail'");
$id_activ = mysql_fetch_array($activ);
$activation = md5($id_activ['id']);
$subject = "Registration Confirmation";
$message = "Hello, $db_fName! Thank you for filling Registration Form for Participation in 23rd Annual Meeting of the Baltic Sea Region Program on Occupational Health and Safety on XIV OHRNC-2017 site.\nIn order to confirm the entered information, you need to activate your e-mail.\nTo activate your e-mail, click the link: http://congr2017/eng/balt-reg-safe-meet/form/activation.php?email=".$db_eMail."&act=".$activation."\nSincerely, Administration http://congr2017/";//содержание сообщение
mail($db_eMail, $subject, $message, "Content-type:text/plain; Charset=windows-1251\r\n");
?>

<?
include 'db.php';
if(isset($_GET['act']) AND isset($_GET['email'])) {
$act = $_GET['act'];
$act = stripslashes($act);
$act = htmlspecialchars($act);
 
$db_eMail = $_GET['email'];
$db_eMail = stripslashes($db_eMail);
$db_eMail = htmlspecialchars($db_eMail);
}
else{
exit('<p class="cntr rd">You have entered the&nbsp;page without confirmation code!</p>');
}
$activ = mysql_query("SELECT id FROM participants WHERE email='$db_eMail'"); //извлекаем идентификатор пользователя с данным e-mail
$id_activ = mysql_fetch_array($activ);
$activation = md5($id_activ['id']);
$db_authDate = date("Y-m-d H:i:s");
if ($activation == $act) {//сравниваем полученный из url и сгенерированный код
mysql_query("UPDATE participants SET auth='1', auth_date='$db_authDate', hash='$activation' WHERE email='$db_eMail'");
echo '<p class="cntr">Your e-mail address has been successfully activated on&nbsp;'.$db_authDate.'!<p>';
}
else {
echo '<p class="cntr rd"><span class="btxt">Error!</span> Your account is not activated. Please, try again or&nbsp;contact the&nbsp;site administrator.</p>';
}
?>