why mysql can have two duplicate columns?

Question

As it shows , this table has two duplicate columns , but I tried to create a table and add a duplicate column,failed,how to add a duplicate column like this ?

mysql> select * from tableA limit 10;
+------+-------+------------------+--------+-----------+
| id   | id_no | contno           | ishigh | contno   |
+------+-------+------------------+--------+-----------+
| NULL | NULL  | 8026000161690448 | NULL   | NULL      |
| NULL | NULL  | 8026000168763808 | NULL   | NULL      |
| NULL | NULL  | 8026000163553868 | NULL   | NULL      |
| NULL | NULL  | 8026000168945478 | NULL   | NULL      |
| NULL | NULL  | 8026000160011788 | NULL   | NULL      |
| NULL | NULL  | 8026000159745758 | NULL   | NULL      |
| NULL | NULL  | 1005043000850458 | NULL   | NULL      |
| NULL | NULL  | 8026000160549108 | NULL   | NULL      |
| NULL | NULL  | 8026000163560088 | NULL   | NULL      |
| NULL | NULL  | 8026000158243148 | NULL   | NULL      |
+------+-------+------------------+--------+-----------+
10 rows in set (0.00 sec)

the create table DDL is:

CREATE TABLE `tableA` (
  `id` int(11) DEFAULT NULL,
  `id_no` varchar(100) DEFAULT NULL,
  `contno` varchar(100) DEFAULT NULL,
  `ishigh` varchar(100) DEFAULT NULL,
  `contno` varchar(1024) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)