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.
Thanks for the hints. I proably have an older s9y, I had to edit include/functions_rss.inc.php, for those who need that too. I requested my re-entry to the planet now.
My advice when tweaking with blog software or migrating between such things: tune down the number of posts that your feeds carry, massively. Flooding planets with 1-2 posts is only a minor irritation: but with 12 or more it becomes a real inconvenience.
Is that kind of a recursive april’s fools joke, which is still running?
Well, I especially like this posting here reapearing. I guess it is *really* fixed know ;-)
I have no explanation for that! Comments on other stories aren’t causing dupes. I don’t know why this one re-appeard.