mysql插入时防止重复记录

INSERT INTO crm_score_exchange_coupon_log(crm_user_score_mall_list,crm_user_member,exchange_date,cost_score,remark,exchange_type)
SELECT {$id},{$crm_user_member},NOW(),'{$score}','{$remark}','{$req_from}' FROM DUAL WHERE NOT EXISTS(
SELECT * FROM crm_score_exchange_coupon_log WHERE crm_user_member={$crm_user_member} AND crm_user_score_mall_list={$id} )