It appears to be related to the duplicate element issue that was described here and quickly fixed.
Mark Pilgrim writes:
... the spec has absolutely nothing to say about the case where a feed contains two elements that mean the same thing. We had been around and around on this issue, and no one could tell us what to do, so we made a judgement call. ... as far as we could tell, we were the only 2 people in the world who gave a shit. I didn't see anyone else even bothering enough to care until a few weeks ago.
Once SSF-DEV set themselves up as the group who cared about this sort of RSS minutia, we immediately deferred to their judgement. Thank God *someone* cares; ... I hope they clarify the rest of the manure that is the RSS spec; the validator will be the first beneficiary of their efforts, I promise you.
This duplicate-element issue sounds like an honest mistake. Pilgrim and Sam Ruby tackled the thankless job of writing a validator for a loosely specified protocol without many volunteer eyeballs, from the looks of the project's mailing list. As one of the only people poking around OPML, I can sympathize.