[PATCH 3/3] doc: add some documentation about duplicate files.

[David Bremner <david@tethera.net>]

This is mainly motivated by the new emacs-UI features to select
duplicates, but hopefully it will help demystify things for users of
the CLI as well.
---
 doc/man1/notmuch.rst  | 17 +++++++++++++++++
 doc/notmuch-emacs.rst | 25 +++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/doc/man1/notmuch.rst b/doc/man1/notmuch.rst
index 1fec4ee5..4b146e41 100644
--- a/doc/man1/notmuch.rst
+++ b/doc/man1/notmuch.rst
@@ -155,6 +155,23 @@ equivalent:
    notmuch --config:alt-config config get user.name
    notmuch --config alt-config config get user.name
 
+.. _duplicate-files:
+
+DUPLICATE MESSAGE FILES
+=======================
+
+Notmuch considers the :mailheader:`Message-ID` to be the primary
+identifier of message. Per :rfc:`5322` the :mailheader:`Message-ID` is
+supposed to be globally unique, but this fails in two distinct
+ways. When you receive copies of a message via a mechanism like
+:mailheader:`Cc` or via a mailing list, the copies are typically
+interchangeable. In the case of some broken mail sending software, the
+same :mailheader:`Message-ID` is used for completely unrelated
+messages. The options :option:`search --duplicate` and :option:`show
+--duplicate` options provide the user with control over which message
+file is displayed. Front ends will need to provide their own
+interface, see e.g. :any:`emacs-show-duplicates`.
+
 ENVIRONMENT
 ===========
 
diff --git a/doc/notmuch-emacs.rst b/doc/notmuch-emacs.rst
index e69bd23d..162f62a4 100644
--- a/doc/notmuch-emacs.rst
+++ b/doc/notmuch-emacs.rst
@@ -431,6 +431,31 @@ in :ref:`notmuch-search`.
 ``c ?``
     Show all available copying commands
 
+.. _emacs-show-duplicates:
+
+Dealing with duplicates
+-----------------------
+
+If there are are multiple files with the same :mailheader:`Message-ID`
+(see :any:`duplicate-files`), then :any:`notmuch-show` displays the
+number of duplicates and identifies the current duplicate. In the
+following example duplicate 3 of 5 is displayed.
+
+.. code-block::
+   :emphasize-lines: 1
+
+    M. Mustermann <max@example.com> (Sat, 30 Jul 2022 10:33:10 -0300) (inbox signed)      3/5
+    Subject: Re: Multiple files per message in emacs
+    To: notmuch@notmuchmail.org
+
+To display a different message file with the same
+:mailheader:`Message-ID`, the user can run the command
+:emacscmd:`notmuch-show-choose-duplicate`, by default bound to `%`.
+
+.. emacscmd:: notmuch-show-choose-duplicate
+
+   |docstring::notmuch-show-choose-duplicate|
+
 .. _notmuch-tree:
 
 notmuch-tree
-- 
2.35.2