Question [Solved] prevent name reuse

@chuck 

Here is the code provided by our developers: 

add_filter('preprocess_comment', function ($commentdata) {
    global $wpdb;
    $sql = "SELECT `comment_author_email` FROM `{$wpdb->comments}` WHERE `comment_author` = %s LIMIT 1";
    $author_email = $wpdb->get_var($wpdb->prepare($sql, $commentdata['comment_author']));
    if (!empty($author_email) && $commentdata['comment_author_email'] !== $author_email) {
        wp_die('name already in use by someone else, please choose a different name.');
    }
    return $commentdata;
});

Insert the code into your WordPress active theme's function.php file.