Enternal Server Error

Error in class CDbDriver, function Select()

Duplicate entry '24840' for key 'group_key' Q: SELECT href,date,title,COUNT(*) AS count,mess_id FROM( SELECT f.id AS mess_id, f.first_id, eh.href AS href, eh.date AS date, ( SELECT eh.about FROM forum AS f2 LEFT JOIN expert_hrefs AS eh ON ( eh.id = f2.href_id ) WHERE f2.level_id='' AND f2.first_id=f.first_id AND eh.is_show =1 ) AS title FROM forum AS f LEFT JOIN expert_hrefs AS eh ON ( eh.id = f.href_id ) WHERE f.first_id IN ( SELECT f3.first_id FROM forum AS f3 LEFT JOIN expert_hrefs AS eh ON ( eh.id = f3.href_id ) LEFT JOIN expert_href_sections AS ehs ON ( ehs.id = eh.section_id ) WHERE eh.is_show=1 AND ehs.group!=100 AND 1 ) ORDER BY f.id DESC ) t GROUP BY first_id ORDER BY mess_id DESC LIMIT 5