I have a blog which is related to Sneakers and from time to time i will post my topics in multiple categories..For example i have one section for just nike and another section for Nike dunks... If i post my topic in the Nike category as well as the Nike Dunk category does this mean i am posting duplicate content and will be penalized for it?
Yes, that would be considered duplicate content. Proper use of robots.txt and robots meta tags can prevent this. In theory, it would be smarter to use one catagory per post only.
Where's the duplicate content? On the "Category" page? Posting the article to multiple categories still only creates one instance of the page at a unique URL - so the article itself is unique and resides at only one spot on your blog. As long as you have different articles in the two categories (i.e. you don't post every article to these two categories), I don't see why it would be duplicate content. There would be some duplication, but one category page as a whole is clearly different from another. If you only care about robots (and this does create a duplicate content penalty), then sure. But people may use categories to browse your site. An article can easily fit in more than one category - and by only placing it in one, your preventing your users from navigating to your content. - Walkere