Database error when trying to upgrade

[Celtic_Mad]

I'm trying to upgrade from vBulletin 3.6.8 but when I go to /install/upgrade.php I get this database error:

Database error in vBulletin 3.6.8:

Invalid SQL:
ALTER TABLE attachment ADD INDEX userid (userid);

MySQL Error   : Duplicate key name 'userid'
Error Number  : 1061
Request Date  : Wednesday, May 28th 2008 @ 07:04:11 PM
Error Date    : Wednesday, May 28th 2008 @ 07:04:11 PM
Script        : http://www.website.com/install/upgrade_300b6.php?step=2
Referrer      : 
IP Address    : xx.xxx.xxx.xx
Username      : 
Classname     : vb_database
MySQL Version : 4.1.22-standard

I've disabled all hacks.

What could the problem be?

[Wayne Luke]

The index already exists. You shouldn't need to run each step more than once.

[Celtic_Mad]

The index already exists. You shouldn't need to run each step more than once.

So how do I skip the step?