Prevent adding users with same email addresses

I am adding new users inside of the Craft controller as part of the frontend user import script.
This is the code:

$user = new User();
$user->pending = true;
$user->username = $email;
$user->firstName = $firstName;
$user->lastName = $lastName;
$user->email = $email;
$user->passwordResetRequired = false;
$user->validate(null, false);
Craft::$app->getElements()->saveElement($user, false);

The problem is that if I try to import two users with the same email addresses they just get imported and that’s obviously something I don’t want to happen. How can I prevent that and silently skip the users with existing email addresses.

I could check the user’s existence by querying it in the database but I would like to avoid any unnecessary db queries because there will be thousands of users imported at the same time. That validation line looks promising but I’m not finding any useful instructions in the docs.