Duplicate unique key error after rollback in mysql

I have a table like this

mysql> describe seudonimos;
+--------------+------------------+------+-----+---------+----------------+
| Field        | Type             | Null | Key | Default | Extra          |
+--------------+------------------+------+-----+---------+----------------+
| id_seudonimo | int(11) unsigned | NO   | PRI | NULL    | auto_increment |
| seudonimo    | varchar(45)      | NO   | UNI | NULL    |                |
+--------------+------------------+------+-----+开发者_Python百科---------+----------------+
2 rows in set (0.02 sec)

Let's assume that it is empty so autoincrement is 0. For example:

SET AUTOCOMMIT=0;
START TRANSACTION;
INSERT INTO seudonimos (seudonimo) VALUES ('Agatha Christie');
ROLLBACK;
SET AUTOCOMMIT=1;

As far as I konw, the rollback does not affect the autoincrement. So if I insert a new value the autoincrement will be 2 instead of 1. But if I try to insert 'Agatha Christie' again, I have the following problem:

INSERT INTO seudonimos (seudonimo) VALUES ('Agatha Christie');

#1062 - Duplicate entry 'Agatha Christie' for key 'seudonimo'

That is not what I expected. I expected this:

+--------------+--------------------+
| id_seudonimo | seudonimo          |
+--------------+--------------------+
|            2 | Agatha Christie    |
+--------------+--------------------+

What's wrong?

You probably use MyISAM engine... Run show create seudonimos; and see.