Does one post on multiple categories affect my SEO?

its fine but content in every post must be unique.

if its relevant no, i believe Google is capable of detecting it.

Each post should be unique. Both when it comes to posts on different pages and posts on the same page, only in different categories.

One post within multiple categories is perfectly fine, as long as it's a good fit. Make sure the post only exists once (only one URL per article).

Best practice is avoid category in your URL.

example.com/article instead of example.com/category/article