WordPress Canonical URLs for Category/PostName Permalinks

When the permalink format is set to %category%/%postname%/ then a post is available under category/postname links for all categories, as well as under all parent/child/postname links.

If you use Yoast SEO and have set a primary category, that becomes the canonical URL that's used for SEO. If you don't then the postname for whatever category has a lower ID (was created first) becomes the canonical URL.