Manual testing revealed that re-setting the headers is necessary
in the "crypt == True" code branch. Thus a conditional fixes the issue.

changeset:   4969:010ce624b320
Fixing duplicated email headers message-id, reply-to, in-reply-to. (part of issue2550869).


Details: 
The testing was done in the following way.
A print statement was added to db_test_base.py:
--- a/test/db_test_base.py      Sun Feb 08 00:35:41 2015 +1100
+++ b/test/db_test_base.py      Tue Mar 10 21:16:50 2015 +0100
@@ -2050,6 +2050,7 @@
                 messages = [m], nosy = [db.user.lookup("fred"), john])

             db.issue.nosymessage(i, m, {})
+            print str(res)
             res.sort(key=lambda x: x['mail_to'])
             self.assertEqual(res[0]["mail_to"], ["fred@example.com"])
             self.assertEqual(res[1]["mail_to"], ["john@test.test"])
then
  python run_tests.py any PGP
saving the output list in file y, making a python statement out of it:
a=
for i,v in enumerate(a):
 for j,w in enumerate(v):
   print j,w
running python file >v1
and comparing the variants manuelly (e.g. by diff or kompare).

The remaining test failures are other headers now,
thus I've created a new issue for it 2550877.