Thanks to some assistance from Garvin (lead Serendipity developer), it looks like the bug that Planet dislikes so intensely is indeed the 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 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 tag that Serendipity is putting in there.
Sorry about that — hopefully it is fixed now.