Over in https://bugs.debian.org/812003
On Tue 2016-01-19 23:26:21 -0500, Daniel Kahn Gillmor wrote:
> When composing a message with an Fcc: line in emacs message-mode, if
> the message has any cryptographic operations set on it
> (e.g. mml-secure-message-sign-pgpmime, or
> mml-secure-message-encrypt-pgpmime), then those operations are done
> distinctly for the message sent via the mail system and the message
> stored in Fcc.

For users of notmuch-emacs, they may prefer to avoid Fcc: and instead
use somthing like the following:

 (defun notmuch-fcc-buffer ()
  (shell-command-on-region (point-min) (point-max) "notmuch insert"))

and then set message-send-mail-hook to refer to notmuch-fcc-buffer.
then probably you'll also want to set notmuch-fcc-dirs to nil.

Alternately, notmuch-emacs might want to use something like this and
route around the entire message-do-fcc function in message.el if
message.el does not address the problem.

        --dkg