Showing duplicate posts

Hey Guys,

Say I have a blog where the posts belong to two categories. Some posts can belong to both. The way I'm thinking to show the posts is something like this:

/blog : shows all the blog posts

/blog/category-1 : shows posts that only belong to the first category.

/blog/category-2: ...

​

Given that some posts will show up under two urls, is it counted as duplicates from the SEO point of view and hence my site will be penalized?

​

Thanks