I think Digg.com has a pretty straightforward way to deal with this. If you try to post a new entry, digg seems to check from
a) the content of the landing page
b) other similar descriptions on the digg site
c) the title of your posting
to determine, whether there is a similar posting already on the site. A similar approach could work for Yedda, too, I think.
Best, Bjoern
