Category Template – can’t implement

I followed the documentation on creating a category template, I even used the default theme to test…Keep getting this error:
Fatal error: Cannot redeclare get_category_children() (previously declared in /var/www/myhosts/mysite.com/httpdocs/blog/wp-includes/category-template.php:3) in /var/www/myhosts/mysite.com/httpdocs/blog/wp-content/themes/default/category-7.php on line 3