I was double-charged

Please, submit a ticket to the technical support team and send us the details of the payment on question. 

Android - Provide us the Transaction ID from the mail that was sent to you by Google with the payment (the Transaction ID should have a GPA.****-****-****-***** format). You can find your Transaction ID in the Google Payments Center, clicking on the specific payment.

iOS - Send the screenshot of the Transaction from the email that was sent to you by AppStore with the payment. You can also find the information about your purchases in your iTunes account: iTunes Store → View Account → Purchase History → See All.