Duplicate content with the same URL, google authorship

The issue is not in how your site is coded or operated, but in how Google decides to show their search results. When the homepage is pulled up, sometimes they’ll show recent posts below it, but often won’t show your author information next to it, because the same author has already been shown above. So there’s nothing that you can do to change this behaviour. It sounds like your authorship markup is set up correctly, which is about all you can do.