SELECT firstname, name, description, reg_ip FROM `students` t1
where exists (select reg_ip from `students` t2 where t1.reg_ip=t2.reg_ip and t1.id<>t2.id)

CREATE TABLE IF NOT EXISTS `students` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `firstname` varchar(20) NOT NULL,
  `name` varchar(10) NOT NULL,
  `description` text NOT NULL,
  `reg_ip` varchar(15) NOT NULL
  PRIMARY KEY (`id`)
) ENGINE=InnoDB

SELECT firstname, name, description, reg_ip 
FROM students
WHERE reg_ip IN
  (
  SELECT reg_ip 
  FROM students
  GROUP BY reg_ip
  HAVING COUNT(id) > 1
  AND TRIM(GROUP_CONCAT(description SEPARATOR '')) <> ''
  )
;