function comment_exists( $comment_author, $comment_date, $timezone = 'blog' ) {
  global $wpdb;

  $date_field = 'comment_date';
  if ( 'gmt' === $timezone ) {
    $date_field = 'comment_date_gmt';
  }

  return $wpdb->get_var(
    $wpdb->prepare(
      "SELECT comment_post_ID FROM $wpdb->comments
			WHERE comment_author = %s AND $date_field = %s",
      stripslashes( $comment_author ),
      stripslashes( $comment_date )
    )
  );
}