If I organize my site's content into folders, I can access it via both of these URLs: http://domain.com/folder/ http://domain.com/folder/index.php If people started using both of these URLs, would Google see these as two different things and penalize me for duplicate content, or are they smart enough to know it's the same thing?
Matt Cutts duplicate content question However technically it is the same content on 2 different URLs and you should redirect one to the other IMO.