Tim Bray thinks that technology will solve the ID problem in feeds:
With Atom, every entry is required to have both a universally-unique ID and a field called updated, the time-stamp when the publisher thinks it last changed. That means that if ever I see the same entry twice, that’s a bug! There’s no wiggle room, somebody’s breaking a rule and I can track it down and harass them until they fix it.
Yeah, right. This assumes a lot - like every content aggregator in the food chain preserving id tags. RSS 2.0 has GUIDs already, and I can tell you, not all content aggregators preserve them - many of them just go ahead and create their own. Atom will not wave a magic wand and fix bad practice.
The only way to deal with ths problem is a system like NNTP, but I don't see that happening.