Unique Email Address

    // check for duplicate email address
    $sql = "SELECT internalKey FROM ".$modx->getFullTableName("web_user_attributes")." WHERE email='$email'";
    if(!$rs = $modx->db->query($sql)){
        $output = webLoginAlert("An error occured while attempting to retreive all users with email $email.").$tpl;
        return;
    } 
    $limit = $modx->db->getRecordCount($rs);
    if($limit>0) {
        $row=$modx->db->getRow($rs);
        if($row['internalKey']!=$id) {
            $output = webLoginAlert("Email is already in use!").$tpl;
            return;
        }
    }