doc: add some documentation about duplicate files.

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.

1 files changed, 17 insertions, 0 deletions

diff --git a/doc/man1/notmuch.rst b/doc/man1/notmuch.rst
index 1fec4ee5..c488f12a 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. the Emacs front-end :any:`emacs-show-duplicates`.
+
 ENVIRONMENT
 ===========