Thanks to some assistance from Garvin (lead Serendipity developer), it looks like the bug that Planet dislikes so intensely is indeed the <slash:comments> tag. I don’t believe this is a bug in Serendipity bug rather in Planet.
(There is still, IMHO, a pubDate bug in Serendipity, but it appears to be unrelated)
So, the dupes you were seeing from me really are gone now. I hope.
The fix is to edit the file templates/default/feed_2.0.tpl and remove the line that provides the <slash:comments> tag.
BTW, seems that Planet Haskell also ran afoul of this.
It seems that some of my posts are appearing on Planet Debian multiple times, and others not at all. I think the problem is a strange combination between subtle bugs in Planet and Serendipity. The SF bug report for Serendipity is here. I think the workaround should fix this.
Either that, or Planet is somehow taking offense as the <slash:comments> tag that Serendipity is putting in there.
Sorry about that — hopefully it is fixed now.
Well, finally! I’ve switched to Serendipity!
The best part: comment spam blocking that works. So to those of you that had trouble commenting on this site… try again. It should just work! (And I’m sorry you had to put up with the hassle so long.)
Serendipity is a very nice system. I’m glad I chose it. And the support from Garvin and the others has been great too.
I’m running 1.0beta2 and have found a few bugs, but nothing serious.
As I mentioned before, I wrote a tool in Haskell to convert posts, comments, and categories from Drupal to Serendipity. If you use darcs, you can fetch it with:
darcs get --partial http://darcs.complete.org/unmaintained/blogcvt
The darcs-impaired may download a tarball.