The code causing the header violation comes from Ralf in the following commit.
In the python 2.7.6 I am testing even rev:4542 never did this correctly, so I am not
sure why Ralf ever added it  in the first place?

Ralf, do you remember why you wrote the lines? :)

changeset:   4542:46239c21a1eb
user:        Ralf Schlatterbeck <schlatterbeck@users.sourceforge.net>
date:        Fri Oct 07 18:04:00 2011 +0000
files:       CHANGES.txt roundup/roundupdb.py test/db_test_base.py test/gpgmelib.py 
test/test_mailgw.py
description:
Sending of PGP-Encrypted mail to all users or selected users (via roles)...

...is now working. (Ralf)



Digging further I found that the email comparison function in test_mailgw.py
does not catch this, because it only compares one of the duplicated headers.
And that roundup still uses the deprecated rfc822 module. We should probably
test if the email module does not do this better anyway.