James Holderness:"RSS 2.0 has a guid element that fits the bill perfectly, but it’s not a required element and many feeds don’t use it. As a result, aggregator authors are left guessing, and nearly every one of them guesses differently.

I can’t say for sure what algorithms applications are using, but after running 150 tests on more than 20 different aggregators, I think have a fair idea how many of them work.

As you would expect, for most the guid is considered the key element for determining duplicates. This is pretty straightforward. If two items have the same guid they are considered duplicates; if their guids differ then they are considered different.

If a feed doesn’t contain guids, though, aggregators will most likely resort to one of three general strategies – all of which involve the link element in some way."

And hence, Atom has an id element. File under Atom-is-a-cleaned-up-RSS2.0.