Save duplicates list

[Harry Putnam <reader@newsguy.com>]

Discussion of duplicate handling in gnus info pages seems to indicate that
setting `gnus-save-duplicate-list'  non-nil (t) will cause gnus to
save a list of duplicates on startup.

It even mentions that this will slow down startup, so seems to indicate
that gnus will process each group looking for duplicates.  

I have a number of duplicates bearing the Warning thing gnus stamps on
them, yet I get no list of them.  Only a file under ~/News/suppression
that says

   (setq gnus-dup-list 'nil)
(EOF -ed HP)

Hard to tell at the info url below if it means it only works on
articles you actually read or what.  Seems not to, what with the talk
of effecting startup. Or if you have to have told gnus to suppress
duplicate as well

At: info://Gnus/Duplicate+Suppression
(In part)
   
   `gnus-save-duplicate-list'
        If non-`nil', save the list of duplicates to a file.  This will
        make startup and shutdown take longer, so the default is `nil'.
        However, this means that only duplicate articles read in a single
        Gnus session are suppressed.