Check if field value exists / prevent duplicate submissions

The question has been already asked multiple times without a proper answer so it seems that it is a highly requested feature and i try it again…

Im looking for a way to check if there is already a submission with the same email adress for example and show an error / prevent saving again to the database if so.

I came across a possible solution here but it does’nt seem to work!?

add_action( 'wpcf7_before_send_mail', 'check_email' );

function email_already_in_db ( $result, $tags ) {
    // Retrieve the posted form
    $form  = WPCF7_Submission::get_instance();
    $form_posted_data = $form->get_posted_data();

    // Get the field name that we want to check for duplicates.
    // I added 'unique' to the beginning of the field name in CF7
    // Checking for that with preg_grep
    $unique_field_name = preg_grep("/unique(\w+)/", array_keys($form_posted_data));

    // $unique_field_name comes back as array so the next three lines give us the key as a string
    reset($unique_field_name);
    $first_key = key($unique_field_name);
    $unique_field_name = $unique_field_name[$first_key];

    // Check the form submission unique field vs what is already in the database
    $email = $form->get_posted_data($unique_field_name);
    global $wpdb;
    $entry = $wpdb->get_results( "SELECT * FROM wp_cf7_vdata_entry WHERE name LIKE '$unique_field_name' AND value='$email'" );

    // If already in database, invalidate
    if (!empty($entry)) {
      $result->invalidate($field_name, 'Your email: '.$email.' already exists in our database.');
      }
    // return the filtered value
  return $result;
}

https://stackoverflow.com/questions/29504337/how-do-i-check-if-email-submitted-via-contact-form-7-exists-in-my-database