HMRC have over calculated my tax refund and then refunded it twice

I filed my tax return, having worked out what my tax overpayment refund should be. HMRC refunded this amount direct into my bank account in July. Then I receive a cheque through the mail for the original repayment amount plus a £1,600! No where on any of the help pages is there information on how to refund the money back to HMRC that they have over paid me. Has anyone else had this issue? I would love to keep it, but it's not mine. What to do.

Hi,
To confirm whether this additional amount is correct, and why it has been refunded to you, we would need to review your record.
Contact us by webchat or phone via Self Assessment: general enquiries to allow us to do this.
Thank you.