Permalink Custom Structure "Doubles" the URL

I want to change the permalinks from
http://www.myblog.com/category/category_name/
to
http://www.myblog.com/category_name/
so I used the custom structure %category% (with / and without) in the Category Base optional field, but the URL becomes
http://www.myblog.com/category_name/category_name/

The same happens if I add /%postname%/ in Common Settings for post URLs.
http://www.myblog.com/post_name/
becomes
http://www.myblog.com/post_name/post_name/

A long ago I removed /%postname%/ and checked “http://www.myblog.com/sample-post/” which worked, but now I need to fix this for categories and I don’t know what to do to make it work.