Email notifications fail if duplicate headers exist

Yes, it is a standard install; AFAICS there is only one container running. (Bring up a new VM, repoint DNS, apt update, apt dist-upgrade, reboot, git pull, ./discourse-setup, Web-based setup, upload backup, restore backup, re-enable mail, see the errors again.) Note that the old install was able to email the link to the backup, and sending test emails still works in the new install — it’s seemingly only posting-related mails that fail.