So I can't be 100% sure, but it looks like your duplicate tickets should be different tickets.
They appear to have been replied to by someone who is:
- not the original ticket owner.
- not a collaborator on the ticket.
- the Ticket number in the subject does not appear to be an existing ticket
You only gave us two tickets that appear to be duplicates, and the From is both different on both of them, which supports that theory.
Alternatively, if they are replying to a Closed ticket, and you have not allowed tickets to be re-opened on response then a new ticket would be the appropriate action for the system to take.