Same issue it's always had, that most websites are configured to accept more than one unique URL, with minor alterations. Often it's something like "?source=twitter" on the end, or "#top". In the case you linked to, one has a trailing slash and the other doesn't.
http://news.ycombinator.com/item?id=2510757 http://news.ycombinator.com/item?id=2511398