Last I checked, identical URLs are automatically prevented. But the duplication becomes a problem when people decide to submit blog posts, etc. instead of original articles. There is (usually) only one "best" URL for a story.
Proposed guideline: submit the "printer-friendly" version?
e.g.
http://example.com/2009/23137321.html?from=rss
http://example.com/2009/23137321.html
They are basically both the same article.