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.