Will Categories and Sub-categories considered as duplicate content ? how it affects SEO ?

Just wanted to clear a doubt. I own a blog where I post regularly and gets a decent traffic. I have a main category (eg : abc) and sub-categories under main category (abc). So when I blog a new post, I select the sub-category and publish them.

All my categories and sub-categories are indexed. I allowed the categories to be indexed in Google.

Now when viewing my blog, the blog post which I marked in a sub-category appears both in Main category (example.com/category/abc) and also the post appears on sub-category (example.com/category/abc/xyz).

So will it be considered as duplicate anyway ?

If some one would help, will be great. thanks in advance.

Also adding to it, Will marking a blog post in multiple categories, affect in anyway in search ?

Yes its duplicate content. You should add the robots 'noindex' 'follow' to every page that has the duplicate content - apart from the main article page obviously. This tag means google will still crawl that page and all links on it, but will not index it in the search results.

more info here: http://www.robotstxt.org/meta.html

thanks. Do you have answer for my second Q - adding multiple categories ? also the third one.

Same Answer for the 2nd question too. Basicly any page that has duplicate content on it is just that, duplicate content - so could effect your ranking. You dont want any duplicate content to be able to get indexed, so use noindex, follow or the canonical tag.

Dont know about your third question, possible, but you wont know for sure until you drop.