duplicate column name after update

I was updating my captcha module from 6.x-2.2 to 6.x-2.4 and got this message right after the update on the update screen in red:

Duplicate column name 'token' query: ALTER TABLE captcha_sessions ADD token varchar(64) in /home/my domainname/domains/mydomainname/public_html/update.php on line 69.

There seems to be no problem but I would like to know why I got this message.
I didn't double click on anything, just update modules as I normally do.

Cheers!