Add additional e-mail addresses to WordPress comment notifications

gistfile1.php

<?php

function filter_comment_notification_headers( $message_headers )
{
    $notify_other_people = array(
        '"John Smith" <john.smith@email.address>',
        '"Jane Smith" <jane.smith@email.address>'
    );

    $message_headers .= "\n" .
        'CC: ' . implode(', ', $notify_other_people );

    return $message_headers;
}
add_filter('comment_notification_headers', 'filter_comment_notification_headers');