The availability of refunds is determined by the organizer of the event and outlined in the site's Terms of Service. This information is always available via a link at the bottom of every page on the site. This is also found as a link in the ticket checkout before completing an order (see image).
If you believe you have made a duplicate purchase, please reach out immediately to support to confirm the duplicate and provide a refund for the unintentional purchase. You can reach support via the support or chat option on the bottom right of the event's page.