convertion to utf-8

[Pooly]

Hi,

I'm trying to convert my tables to UTF8 but I'm getting the following error:
ERROR 1062 (23000): Duplicate entry 'Zorglüb' for key 1

Not too sure why I'm getting this error since the current (latin1) data are:
mysql> select * from topics_lookup where label like 'Zor%';
+----------+----------+------+
| label    | topic_id | main |
+----------+----------+------+
| Zorglub  |       72 |    0 |
| Zorglüb  |       72 |    1 |
+----------+----------+------+
2 rows in set (0.00 sec)

There is a unique index on label, however the 2 data are different.

Any ideas ?
Thanks,