The technical problem most of the times is how the MTA handles these emails.
Are you keeping emails on the mail server or are they removed after fetching?
If they’re kept, it’s possible that -due to the same message ID- Zammad imported only one of the two mails (and that’s gambling mostly).
Zammad as a software hates this use case, because it always tries to have one comminucation ticket over several ones. This is also the case if you… let’s say answer to a ticket with a ticket number and CC another group - the CC will be “lost”.