emember, no matter how much we might sometimes like to think otherwise, Google is just a dumb computer. The GoogleBot crawls the web, reads the pages content and then stores them in a big database.
OK there is more to it to that, but essentially that is it. So when the GoogleBot comes across the same content under different URLs it doesn’t know which to present to people in the search results.
Equally you might want to try and increase your search engine rankings by copying the same (or very similar) content onto many different pages of your website in a (misguided) effort to try and increase your relevancy to your chosen keywords.
Also if you think about it from a non-Google perspective, if you were building a website there are very few legitimate reasons that you would put the same content on several URLs.
All of these reasons together add up to one conclusion, Duplicate Content is bad.