This is especially relevant on Feedbase,
where updates to RSS/Atom entries are recorded as new
articles that supersede previous ones.
If you don't care about updates, down-scoring articles with
Supersedes: is a nice solution.
Newsreaders, such as Gnus, can only do that effectively on the fields returned by the XOVER command in NNTP, so while somebody figured out how to do it by making Gnus fetch headers for all articles, it was slow.
After having done that, I still wasn't able to get Gnus to score on the new field.
Tracing through the Emacs lisp code with
M-x edebug-defun didn't
make me any
to the problem, it looked like it was comparing with "nil" rather than
the value I knew XOVER was returning.
The next day I tried tracing some more, and finally realized that I was missing the name of the header in the information I added to XOVER!
After fixing that, this scoring rule worked as expected:
(("extra" ("." -1000 nil r "Supersedes")))