Povedal by som ze si otestujes navratovu hodnotu volania $db->exec($check_exist) ktore das za ten riadok $check_exist = "SELECT * FROM db_users WHERE email='$email'";
T.j. nieco taketo
if($db->exec($check_exist))
echo "Uzivatel uz existruje <br>";
else
{
$register = "INSERT INTO db_users(id, email, password, date, ip_adress, system, language) VALUES ('', '$email', '$password', '$date', '$ip_adress', '$system', '$language')";
$db->exec($register);
}