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gnus: Duplicates

 
 6.4.11 Duplicates
 -----------------
 
 If you are a member of a couple of mailing lists, you will sometimes
 receive two copies of the same mail.  This can be quite annoying, so
 ‘nnmail’ checks for and treats any duplicates it might find.  To do
 this, it keeps a cache of old ‘Message-ID’s:
 ‘nnmail-message-id-cache-file’, which is ‘~/.nnmail-cache’ by default.
 The approximate maximum number of ‘Message-ID’s stored there is
 controlled by the ‘nnmail-message-id-cache-length’ variable, which is
 1000 by default.  (So 1000 ‘Message-ID’s will be stored.)  If all this
 sounds scary to you, you can set ‘nnmail-treat-duplicates’ to ‘warn’
 (which is what it is by default), and ‘nnmail’ won’t delete duplicate
 mails.  Instead it will insert a warning into the head of the mail
 saying that it thinks that this is a duplicate of a different message.
 
    This variable can also be a function.  If that’s the case, the
 function will be called from a buffer narrowed to the message in
 question with the ‘Message-ID’ as a parameter.  The function must return
 either ‘nil’, ‘warn’, or ‘delete’.
 
    You can turn this feature off completely by setting the variable to
 ‘nil’.
 
    If you want all the duplicate mails to be put into a special
 “duplicates” group, you could do that using the normal mail split
 methods:
 
      (setq nnmail-split-fancy
            '(| ;; Messages duplicates go to a separate group.
              ("gnus-warning" "duplicat\\(e\\|ion\\) of message" "duplicate")
              ;; Message from daemons, postmaster, and the like to another.
              (any mail "mail.misc")
              ;; Other rules.
              [...] ))
 Or something like:
      (setq nnmail-split-methods
            '(("duplicates" "^Gnus-Warning:.*duplicate")
              ;; Other rules.
              [...]))
 
    Here’s a neat feature: If you know that the recipient reads her mail
 with Gnus, and that she has ‘nnmail-treat-duplicates’ set to ‘delete’,
 you can send her as many insults as you like, just by using a
 ‘Message-ID’ of a mail that you know that she’s already received.  Think
 of all the fun!  She’ll never see any of it!  Whee!
 

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