Override user registration function
/**
 * Implements hook_form_BASE_FORM_ID_alter().
 */
function mymodule_form_user_register_form_alter(&amp;$form, &amp;$form_state, $form_id) {

  array_unshift($form['#validate'],'my_first_custom_validation' );
  $form['#validate'][] = 'my_last_custom_validation';

}

function my_first_custom_validation(&amp;$form, &amp;$form_state){
  if(isset($form_state['values']['mail'])){
    if($user = user_load_by_mail($form_state['values']['mail'])){
      rules_invoke_component('rules_send_email_to_user', $user);
    }
  }
}



Hook into register form using hook form alter
Add custom validation handler to the top of submit handlers (must execute first before any other submit handler )
In the handler function  try to load user by email using user_load_by_mail
If true send the email using drupal_mail or rules then redirect to same page or any page to stop the execution.


The only problem with this code that the validation error still showing , I could not find way to stop drupal from execute the rest of validation handlers