[wp-hackers] plugin activation help

[Rich Pedley]

I'm getting a couple of people post about some issues when activating 
my plugin eShop.

Fatal error: Cannot redeclare add_user() (previously declared in 
C:\Inetpub\vhosts\ellcee.com\httpdocs\blog\wordpressDEV\wp-admin\includes\user.php:23) 
in 
C:\Inetpub\vhosts\ellcee.com\httpdocs\blog\wordpressDEV\wp-admin\includes\user.php 
on line 44

&

Fatal error: Cannot redeclare add_user() (previously declared in 
C:\inetpub\vhosts\mydomain.com\httpdocs\store\wp-admin\includes\user.php:23) 
in 
C:\inetpub\vhosts\mydomain.com\httpdocs\store\wp-admin\includes\user.php 
on line 44


Obviously I can't replicate this myself.

Any ideas on what would be causing this? Or tips on how to help them 
without access to anything at their end?

Any help appreciated.

Rich