Been there before

I ran across this problem in BottomFeeder awhile ago, and had exactly the same problems that Dare had:

I stumbled across a feed that used the same link for each item from a given day; the Cafe con Leche RSS feed . This meant that RSS Bandit couldn't differentiate between items posted on the same day. This was particularly important when tracking what items a user has read or whether an item has already been downloaded or not. I should have pinged the owner of the feed to point this problem out but instead I decided to code around this issue by using the combination of the link and title elements for uniquely identifying items. This actually turned out to be worse.

Yes, I've had that problem, and it's one of the reasons that differentiating items in RSS is non-trivial. There are blogs out there that don't include GUIDS or links - yes, odd as that sounds, I've seen it. If someone can create weird XML, they will create weird XML...