语法：
DELETE FROM 表名
    WHERE id NOT IN (
        SELECT temp.min_id FROM (
            SELECT MIN(id) min_id FROM 表名
                GROUP BY 字段名
            )AS temp
    );

备注：id：也是表字段

实例：
DELETE FROM pro_completed_table
    WHERE id NOT IN (
        SELECT temp.min_id FROM (
            SELECT MIN(id) min_id FROM pro_completed_table
                GROUP BY md5
            )AS temp
    );