The complete guide to duplicate content
From Yoast.com
In doing my work as an SEO, I constantly encounter people who do not “get” the concept of duplicate content. It’s as old as the search engines themselves, and SEO’s have been fixing it since day one, but we apparently still need to do more about it. That’s why I decided to take some time and write a guide to help you learn what it is, how to identify and how to solve it: Duplicate content: causes and solutions