I have a problem. I’ve added a category to my permalink, but I’m looking that if I add two categories to one post, WordPress can use url of the two categories, so I’m making duplicated things, so that by the view of SEO is horrible… how can I fix it?
The whole duplicate content issue is worried about way too much. Search engines have been written to take things like this into consideration. WordPress will always do that if you are using the category in your permalink.
But keep this in mind – even though those links work – do those links exist anywhere on your page? If not, there is no way for the search engines to see the alternate links.