A user replied to a ticket that had been resolved. I'd expect their message to be added to the ticket thread, but instead a new ticket was created. What's going on?
Check that the user did not reply from a different email address that is not associated with their account in Deskpro.
If they were definitely using the same address, the user probably did not have had permission to re-open a resolved ticket.
Under Admin > CRM > User Groups, check your settings for the "Can re-open resolved tickets" permission.
If none of the user's groups grants the permission to re-open resolved tickets, their message may be rejected or accepted as a new ticket.