DELETE 
  FROM users
 WHERE userid IN (SELECT a.userid
                    FROM users a
                   INNER JOIN
                         (SELECT username, birth
                               , MIN(userid) min_userid
                            FROM users
                           GROUP BY username, birth
                          HAVING COUNT(*) > 1
                          ) b
                      ON a.username = b.username
                     AND a.birth    = b.birth
                     AND a.userid   > b.min_userid
                  )
;