Error when upgrading vb 3.8

[chikuru]

http://www.******.org/vB/install/upg...0b6.php?step=2

Database error in vBulletin 3.8.7:

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

MySQL Error   : Duplicate key name 'userid'
Error Number  : 1061
Request Date  : Friday, January 29th 2016 @ 05:31:00 PM
Error Date    : Friday, January 29th 2016 @ 05:31:00 PM
Script        : http://www.*******.org/vB/install/upgrade_300b6.php?step=2
Referrer      : 
IP Address    : ***.***.***.***
Username      : 
Classname     : vB_Database
MySQL Version : 5.5.47-cll

I tried droping userid and still geting same error