Timothy, are you reading off a a Google or other aggregating page? I suggest you mark the existing messages as read and see if only the new ones pop up. That seems to work for me. Let me know if that works.
I was using Firefox's live bookmark feature, installed by clicking the orange icon with the white stripes at the right of the URL bar. This post is showing on it, but the old ones are too. This live bookmark shows in the "Bookmarks Toolbar". If I put a new one on/subscribe to the feed again for the page, same thing happens.