Ralf Helbing <helbing@monet.cs.uni-magdeburg.de> writes:
   Why do cross posted articles appear in every group they were posted to
   instead of only the first one? In other words: how can I have them
   marked as read in the current group if I read them already in another
   group?
   
   Gnus will read news overview files to build its article lists for a
   group. An (unfortunately) optional field contains information about
   where an article is crossposted to. If this is disabled, then Gnus
   will not be able to properly deal with crossposts.
   
   Get your system administrator to enable Xref in the news overview
   files.
   
   Failing that, if you add
(setq nntp-nov-is-evil t)

   to your .gnus, Gnus will do The Right Thing with respect to cross
   posts at the cost of slower group entry and exit.