Marking cross-posted articles as read ensures that you’ll never have to read the same article more than once. Unless, of course, somebody has posted it to several groups separately. Posting the same article to several groups (not cross-posting) is called spamming, and you are by law required to send nasty-grams to anyone who perpetrates such a heinous crime.
Remember: Cross-posting is kinda ok, but posting the same article
separately to several groups is not. Massive cross-posting (aka.
velveeta) is to be avoided at all costs, and you can even use the
gnus-summary-mail-crosspost-complaint command to complain about
excessive crossposting (see Summary Mail Commands).
One thing that may cause Gnus to not do the cross-posting thing
correctly is if you use an NNTP server that supports XOVER
(which is very nice, because it speeds things up considerably) which
does not include the Xref header in its NOV lines. This is
Evil, but all too common, alas, alack. Gnus tries to Do The Right Thing
even with XOVER by registering the Xref lines of all
articles you actually read, but if you kill the articles, or just mark
them as read without reading them, Gnus will not get a chance to snoop
the Xref lines out of these articles, and will be unable to use
the cross reference mechanism.
To check whether your NNTP server includes the Xref header
in its overview files, try ‘telnet your.nntp.server nntp’,
‘MODE READER’ on inn servers, and then say ‘LIST
overview.fmt’. This may not work, but if it does, and the last line you
get does not read ‘Xref:full’, then you should shout and whine at
your news admin until she includes the Xref header in the
overview files.
If you want Gnus to get the Xrefs right all the time, you have to
set nntp-nov-is-evil to t, which slows things down
considerably. Also see Slow/Expensive Connection.
C’est la vie.
For an alternative approach, see Duplicate Suppression.