It's not the usual case of extra fluff in the URL causing the uniqueness check to fail, they are significantly different and only following the link would show them as the same thing.

Looks like this one is the canonical URL vs the one I posted previously, or he's updated his blog software and the URLs have changed. I would have thought I'd have noticed the redirect.

The uniqueness checker could follow links & redirects to check for duplicates, or maybe simply a note on the submission page along the lines of:

  please search Google for 'site:news.ycombinator.com "article title"' 
  to make sure this hasn't already been posted.