Double payment

Instalment bounced in the 15th on my account and I immediately made a transfer the next day and MFC re-ran another debit 2 days later meaning they now have been double paid for Dec. I sent proof of payment requesting a refund and was advised that refund will be done in 7-10 working days from the 19th. I send a follow today as the refund has not been done and I get stupid responses of how I can't be refunded with useless statements attached that don't mean absolutely nothing to me and I'm pretty sure their debit order is again going to go off again on the 15th. I want that amount back I did not request to pay extra this was an error done by you guys and I want it rectified.