in this article, I'll show you how to check if a user email already exists in Drupal 8.
juste add ->condition('mail', 'username@example.com') in your query.
$ids = \Drupal::entityQuery('user')
->condition('mail', 'username@example.com')
->execute();
Example
/**
* Implements hook_preprocess_block().
*/
function mytheme_preprocess_page(&$variables)
{
$ids = \Drupal::entityQuery('user')
->condition('mail', 'username@example.com')
->execute();
if (!empty($ids)) {
kint("this mail already exists");
} else {
kint("this mail not exists");
}
}
