The issue of duplicate content is a rather contentious one in the world of Search-Engine Optimization. There were for many years – and perhaps still to this day – myths floating round about duplicate content penalties.
Essentially, folks were told that if they produced duplicate content then they would be punished, and many thought that this meant you could not syndicate content, or post the same article on your site and another. Personally I think this was a ploy by Google to deter potential spammers.
You see, back when search engines first started to become popular, it was easy to rank in the top spots for your keywords by stuffing your website full of keywords. The black hat community used the same article on multiple pages and the search engines gave them good rankings. Thankfully, the algorithms are much more sophisticated these days and so are the users...
Via Martin Gysler