$mailadres = $_GET['mail'];
$a = $_GET['order_number'];    

$query = $db->prepare("SELECT * FROM mail_status where mail=:mail AND order_number=:order_number LIMIT 1");
$kontrol = $query->execute(array(":mail"=>$mailadres,":order_number"=>$a));
$kontrol = $query->fetch(PDO::FETCH_ASSOC);

if($kontrol){
     echo "Bu veri zaten var";
}else{
     $ekle=$db->prepare("INSERT INTO mail_status SET
    order_name=:order_name,
    mail_adres=:mail_adres,
    mail_date=:mail_date
    ");
  $insert=$ekle->execute(array(
    'order_name' => $a,
    'mail_adres' => $mailadres,
    'mail_date' => $date
  ));
}