You must be new here!
If [like I do], you browse HN by the "new" section, you'll see this time after time after time after time...
I don't know what's to be done about it. HN don't seem to care and it seems like a huge number of visitors to the site just can't be bothered spending a few seconds to check whether or not a story has already been submitted, before adding their own noise to the signal.
From the website management point of view one difficulty is that; on the intarwebs in general, the vast majority of these stories originate from a single news agency or press release and are then mirrored across countless other 'news' sites. So, in effect the problem increases exponentially: one story source is republished across 20 different websites and then each of those 20 website articles is submitted to HN by 20 different people.