Everyone has had the problem with the permalink structure when using multiple categories on a single post. Using a plugin like WP Category Permalink or Hikari Category Permalink will let you select the desired category for the permalink to solve the problem. HOWEVER, and I don't know if it is plugin related because I deleted the plugin and the problem still persist, WordPress seems to create URLs for all categories selected in the post. For example: when I publish a post with the categories "featured", "pictures", and "WordPress" and select "featured" as the category for the permalink I will get something like this: mywebsite.com/featured/post-name. BUT, without my knowledge, either the plugin or WordPress will create links for the other categories as well. So the post in my example would also be accessible through mywebsite.com/wordpress/post-name and mywebsite.com/pictures/post-name. These all contain the same post, but with 3 different URLs.
This could be very harmful for your SEO because you now have duplicated content on your own website. Could anyone confirm if this is a SEO problem and if the problem exists within WordPress itself or if it is caused by the earlier mentioned plugins. And is someone without the mentioned plugins able to replicate this problem?