Is selecting 2 or more categories in WordPress bad for SEO?

I'm just going through various tutorials i have from various 'Gurus' & i'm getting conflicting advice.
My thought has alsways been that it's bad for SEO to select 2 or more categories for the same post in WordPress on the grounds that it's '2 bites of the cherry' & Google may see it as duplicate content from the same site.
However, i'm using the 'All in One SEO' plugin & have unchecked 'Use Categories for Meta Keywords', also my Permalink structure is '/%post_id%/%postname%/' so that doesn't pick up the Category in the URL.

I have my categories configured so that i have a Parent category which shows on the Home page & various Child categories under each Parent category.
If i select Parent category & a Child category would that adversely affect my onsite SEO?
Thanks.....a bit long-winded to explain.

Short answer: no!

of course not, I am doing the same with my posts.

an article can have more than one categories, the link to the article remains the same, so google does not consider it as duplicate content.