PublicInbox::NNTP::set_nntp_headers($mime->header_obj, $ng, 1, $mid);
is_deeply([ $mime->header('Message-ID') ], [ "<$mid>" ],
'Message-ID unchanged');
PublicInbox::NNTP::set_nntp_headers($mime->header_obj, $ng, 1, $mid);
is_deeply([ $mime->header('Message-ID') ], [ "<$mid>" ],
'Message-ID unchanged');