All right, after a review of
Google's Webmaster Forum I'm quite convinced that robots.txt overrides the sitemap, which is basically described as a "helper" to Google, not a be-all-end-all list.
In addition, Google themselves states in
How can I create a Google-friendly site?:
Things to Avoid
Don't create multiple copies of a page under different URLs. Many sites offer text-only or printer-friendly versions of pages that contain the same content as the corresponding graphic-rich pages. To ensure that your preferred page is included in our search results, you'll need to block duplicates from our spiders using a robots.txt file. For information about using a robots.txt file, please visit our information on blocking Googlebot.
provides info on pattern-matching in robots.txt, and it may be helpful to exclude the duplicate files -- but as they say it's "an extension of the standard, so not all bots may follow it."
There's a lot of other good info at those links, so I hope this helps.
