Apart from the default New ticket email notification (Admin > Workflows > Email notifications > Requester notifications), there might be a Ticket creation automation rule (Admin > Workflows > Automation > ticket creation) that sends an email every time a new ticket is created to the requester. Please check on the reported ticket's Show Activities to see if there was any automation rule executed on that ticket.

You can navigate to the corresponding automation rule by clicking on the rule link for that activity. From within the automation rule, verify if there is an action 'Send email to requester' within the rule. If so, you can remove this action or add another action 'Skip new ticket email notification to the automation rule, to prevent notification email duplication in cases where this automation rule is triggered on tickets.