As a matter of fact, it’s a really bad idea to assign your posts to several categories because it adds up to the duplicate content that you have throughout your site. It may make sense for the user, but not for Google. In case you absolutely have to use a few categories, here’s what you can do as a workaround.
In order to fix the duplicate content issue, you need to go to SEO -> Titles and Metas -> Taxonomies and tick the “noindex,follow” option across Categories. It’ll allow you to remove your category pages from Google’s index, but still pass along Google juice through them.
Alternatively, just assign one post per single category 🙂