Yes, this would be a useful service. When some item becomes really popular it gets linked to from multiple sources in my feeds and I've already seen it because

Yes, this would be a useful service. When some item becomes really popular it gets linked to from multiple sources in my feeds and I've already seen it because it got linked to on Hacker News or twitter. Bonus points for marking feed items as read if its only content is a link to a page I've already seen.