Just send them an email with both your booking numbers, they'll refund as long as the passenger's name is the same.

You are number 104 in the queue. Your waiting time will be approximately 84 minute(s). Thank you for your patience.

Here use this, it's much faster: https://eform.airasia.com/

What happens if I make a double payment or double booking? 
Our systems will not allow you to make the same booking (same name, same payment method, same flight date & time) within 24 hours.

However, if you use a different payment method or you accidentally make the same booking after 24 hours it will result in a double booking (two tickets with different reference numbers). If that is the case, please click this link and fill up the e-form to request for a refund: in "Case Category" select "Feedback" and in "Type" select "Website Booking/Check-In";. Fill up all mandatory fields marked with an "*". You will need to provide the reference number of both of your bookings.