When you open the email in Outlook, go to File - Properties in the Internet-Headers to find the Message-ID.
If you look at two identical emails, is the Message-ID the same or different for both? It's best to post both Message-IDs here.
When an email arrives, can you see in journalctl that this email is delivered twice? The log message should be something like this.
gromox-delivery-queue[15035]: remote=[::1] from=<fail2ban-xxx-grommunio@xxxxx.at> to={monitor@xxxxx.com} return OK, queue-id:1
Question: Which OS, which gromox version, and which virtualization is being used?
