I use Mambo CMS on my sites and I use a component called SEF Advance. It does a great job of creating search engine friendly URL's. My RSS feeds are showing the original Mambo based URL's. On sites that carry my feeds, they have the old URL's -- and for some reason it doesn't get re-written when you click on the link. Should I have any concerns about Google seeing two links to the same content?
Yes, you should be concerned. That's duplicate content and most people agree that you will be penalized. Try to fix it if you can.