Same post in two categories good/bad for SEO?

What do you think about adding the same, one post in two or more different (sub)categories? My site is structured that way so when it comes to categorizing certain posts they usually fit into two subcategories. If you think user-experience, then adding the post to more categories IS better. But is it bad for SEO? What's a good workaround on that?

Is there a plugin for WordPress that deindexes (or nofollows) certain posts from certain categories? So for example when you are writing the post and you add it to two or more categories, it would be really useful to add a "deindex" option to one or more categories for that post only.

Most WordPress themes have an option for NOT repeating the same post on the homepage, don't know about other CMSs

Learn how to use rel canonical. It will solve your issues immediately.

I think it's bad. Creates dupe content in the archives (the excerpt will be the same).

I usually deindex all categories / tags and also the archive to avoid duplicate content.
Yoast SEO can do this.

Nonilol said:

I usually deindex all categories / tags and also the archive to avoid duplicate content.
Yoast SEO can do this.

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Isn't that bad for seo too? What if your category page gets ranked or a user finds that page when searching?