Quoting from the elisp reference:

     For other types (e.g., lists, vectors, strings), two arguments
     with the same contents or elements are not necessarily ‘eq’ to
     each other.

Thanks to "Attic Hermit" for the fix.
---
 emacs/notmuch-address.el | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/emacs/notmuch-address.el b/emacs/notmuch-address.el
index f937e708..64887a43 100644
--- a/emacs/notmuch-address.el
+++ b/emacs/notmuch-address.el
@@ -159,7 +159,7 @@ matching `notmuch-address-completion-headers-regexp'.
 		       #'notmuch-address-expand-name)))
       (when setup-company
 	(notmuch-company-setup))
-      (unless (memq pair message-completion-alist)
+      (unless (member pair message-completion-alist)
 	(setq message-completion-alist
 	      (push pair message-completion-alist)))))
 
-- 
2.14.1

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