MySQL [test]> create table tbl_keyword (
    ->     id int not null auto_increment primary key,
    ->     keyword varchar(256) not null
    -> )
    -> ;
Query OK, 0 rows affected (0.06 sec)
MySQL [test]> insert into tbl_keyword(keyword) values ("aa"),("bb"), ("cc"), ("aa"),("bb");
Query OK, 5 rows affected (0.05 sec)
Records: 5  Duplicates: 0  Warnings: 0

MySQL [test]> show tables;
+----------------+
| Tables_in_test |
+----------------+
| tbl_keyword    |
+----------------+
1 row in set (0.03 sec)

MySQL [test]> desc tbl_keyword;
+---------+--------------+------+-----+---------+----------------+
| Field   | Type         | Null | Key | Default | Extra          |
+---------+--------------+------+-----+---------+----------------+
| id      | int(11)      | NO   | PRI | NULL    | auto_increment |
| keyword | varchar(256) | NO   |     | NULL    |                |
+---------+--------------+------+-----+---------+----------------+
2 rows in set (0.03 sec)

MySQL [test]> alter table tbl_keyword add idempotent_id char(32);
Query OK, 0 rows affected (0.11 sec)
Records: 0  Duplicates: 0  Warnings: 0

MySQL [test]> alter table tbl_keyword add unique index idempotent_id(idempotent_id);
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

MySQL [test]> desc tbl_keyword;
+---------------+--------------+------+-----+---------+----------------+
| Field         | Type         | Null | Key | Default | Extra          |
+---------------+--------------+------+-----+---------+----------------+
| id            | int(11)      | NO   | PRI | NULL    | auto_increment |
| keyword       | varchar(256) | NO   |     | NULL    |                |
| idempotent_id | char(32)     | YES  | UNI | NULL    |                |
+---------------+--------------+------+-----+---------+----------------+
3 rows in set (0.03 sec)
MySQL [test]> select * from tbl_keyword;
+----+---------+---------------+
| id | keyword | idempotent_id |
+----+---------+---------------+
|  1 | aa      | NULL          |
|  2 | bb      | NULL          |
|  3 | cc      | NULL          |
|  4 | aa      | NULL          |
|  5 | bb      | NULL          |
+----+---------+---------------+
5 rows in set (0.03 sec)

MySQL [test]> insert into tbl_keyword(keyword, idempotent_id) values ("aa", md5("aa")), ("bb", md5("bb")), ("cc", md5("cc"));
Query OK, 3 rows affected (0.03 sec)
Records: 3  Duplicates: 0  Warnings: 0

MySQL [test]> select * from tbl_keyword;
+----+---------+----------------------------------+
| id | keyword | idempotent_id                    |
+----+---------+----------------------------------+
|  1 | aa      | NULL                             |
|  2 | bb      | NULL                             |
|  3 | cc      | NULL                             |
|  4 | aa      | NULL                             |
|  5 | bb      | NULL                             |
| 11 | aa      | 4124bc0a9335c27f086f24ba207a4912 |
| 12 | bb      | 21ad0bd836b90d08f4cf640b4c298e7c |
| 13 | cc      | e0323a9039add2978bf5b49550572c7c |
+----+---------+----------------------------------+
8 rows in set (0.03 sec)

MySQL [test]> insert into tbl_keyword(keyword, idempotent_id) values ("aa", md5("aa")), ("bb", md5("bb"));
ERROR 1062 (23000): Duplicate entry '4124bc0a9335c27f086f24ba207a4912' for key 'idempotent_id'
MySQL [test]> select * from tbl_keyword;
+----+---------+----------------------------------+
| id | keyword | idempotent_id                    |
+----+---------+----------------------------------+
|  1 | aa      | NULL                             |
|  2 | bb      | NULL                             |
|  3 | cc      | NULL                             |
|  4 | aa      | NULL                             |
|  5 | bb      | NULL                             |
| 11 | aa      | 4124bc0a9335c27f086f24ba207a4912 |
| 12 | bb      | 21ad0bd836b90d08f4cf640b4c298e7c |
| 13 | cc      | e0323a9039add2978bf5b49550572c7c |
+----+---------+----------------------------------+
8 rows in set (0.03 sec)