Cannot create new category

I never used to have this problem, but for the past couple of months, every time I attempt to add a new Category for a post, the new category does not appear on my current list of categories, and I get the following error message:

Fatal error: Cannot redeclare get_users_drafts() (previously declared in /home/perthpunk/www/theboomtownrap/wordpress/wp-admin/includes/user.php:233) in /home/perthpunk/www/theboomtownrap/wordpress/wp-admin/admin-db.php on line 9

I have done a search in the forums, and have read the other threads by other folk who have not been able to add categories, but none mention getting this fatal error.

This is now driving me nuts. I was hoping upgrading to 2.3.3 would solve the problem, but it hasn’t.

Grateful for any advice, please.

Cheers