Multiple Permalink Structure

Dear forums,

I want to add the post title to my permalink structure within category.php
So if I typ in the the following url into my brower:

/category/%category%/%postname%/

a single post should be displayed but

/%postname%/ should still be available.

Right now wordpress do not find the post if i enter the first url

how can I change the permalink structur within category.php

without changing the general stetting?

Can someone help me please,

many thanks in advance