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#### 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!