Apart from the default New ticket email notification, there might a Ticket creation automation rule that's sending an email every time a new ticket is created. Please open the ticket from that requester and click on the Activities button to see if there was any automation rule executed on that ticket.
You could navigate to the corresponding automation rule by clicking on the rule link for that activity. From within the automation rule, you could verify if the email notification is being sent, this would be available as an action within the rule. You could remove this action or add another action - "Skip new ticket email notification", to that automation rule, which would prevent email duplication in cases where this rule is triggered.