Does one post on multiple categories affect my SEO?

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

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

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

its fine but content in every post must be unique.