Magento has several issues with duplicated content. Of course, you don’t actually have two versions of the same content in your database, however, in the eyes of search engines, two different URLs serving same content are counted as duplicated content and will cause you lots of problems. The most obvious problem is that your page rank is leaking on different versions of virtually the same landing page. You also might end up with having several versions of the same content indexed which is bad in so many ways.