Gnus Manual. Node: Duplicates

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6.3.9: 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-IDs--- nnmail-message-id-cache-file, which is `~/.nnmail-cache' by default. The approximate maximum number of Message-IDs stored there is controlled by the nnmail-message-id-cache-length variable, which is 1000 by default. (So 1000 Message-IDs 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" "duplication 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:")
        ;; 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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