mysql query duplicate data records

1. Scene: a master data table, a plurality of data word table. When a query needs to show the data word table separated by commas

 

  sql statement:

   

		SELECT
    cdu.user_name as user_name,
    cmp.user_id as create_uid,
    cmr.receive_user_id as receive_user_id,
    cmp.id as id, cmp.mesage_title as mesage_title,
    cmr.status as status, cmp.create_time as create_time,
    cmp.message_content as message_content,
    cmp.status as message_status,
    IFNULL(GROUP_CONCAT(DISTINCT cmd.doc_name),'') as docName
    FROM car_message_post cmp
    LEFT JOIN car_message_receiver cmr ON cmp.id = cmr.message_id
    INNER JOIN car_adm_user cdu ON cmp.user_id = cdu.user_id
    LEFT JOIN car_message_doc cmd ON cmd.message_id = cmp.id
    WHERE
    1=1 and cmp.id  = 252
    GROUP BY cmp.id,mesage_title,cmp.create_time,message_content LIMIT 50;

　　

GROUP_CONCAT (DISTINCT cmd.doc_name) need to add a distinct, duplication of the data word table or else