mysql> CREATE TABLE IF NOT EXISTS `color` (
    ->   `id` int(11) NOT NULL AUTO_INCREMENT,
    ->   `value` varchar(255) NOT NULL,
    ->   PRIMARY KEY (`id`),
    ->   UNIQUE KEY `value` (`value`)
    -> ) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;
Query OK, 0 rows affected (0.06 sec)

mysql> INSERT INTO `color` (`id`, `value`) VALUES
    -> (1, 'FF0000'),
    -> (2, '00FF00');
Query OK, 2 rows affected (0.01 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from color;
+----+--------+
| id | value  |
+----+--------+
|  1 | FF0000 |
|  2 | 00FF00 |
+----+--------+
2 rows in set (0.00 sec)

mysql> INSERT INTO `color` (`value`) VALUES ('FF0000')
    -> on duplicate key update value='FF0000';
Query OK, 0 rows affected (0.00 sec)

mysql> select * from color;
+----+--------+
| id | value  |
+----+--------+
|  1 | FF0000 |
|  2 | 00FF00 |
+----+--------+
2 rows in set (0.00 sec)

mysql> replace into color (value) values('FF0000');
Query OK, 2 rows affected (0.00 sec)

mysql> select * from color;
+----+--------+
| id | value  |
+----+--------+
|  3 | FF0000 |
|  2 | 00FF00 |
+----+--------+
2 rows in set (0.00 sec)

mysql> replace into color (value) values('FF0000');
Query OK, 2 rows affected (0.00 sec)

mysql> select * from color;
+----+--------+
| id | value  |
+----+--------+
|  4 | FF0000 |
|  2 | 00FF00 |
+----+--------+
2 rows in set (0.00 sec)