Citibank Credit Card Complaint

Citibank Credit card department,

Billing disputes Pennsylvania

on -/-/-, I meant to send a billpay payment to Citibank for a credit card account, in the amount of - i accidentally send them a payment for -, resulting in an overpayment of $21000.00. I have been promised a refund as of -/-/- and as of today, -/-/-, i STILL have received NO refund. It has been almost 60 days since i first called in for a refund. I have called 5 additional times over that timeframe, each time escalating the lack of refund and Citibank has done NOTHING except promise me a refund within the week and giving me no refund. Twice we have spent over an hour on the phone while they insist on verifying the payment with my bank, which just wasted my time and made me even more upset. At this point i do n't believe that Citibank will give me any refund. I believe that they intend to keep the overpayment. The lack of refund is unethical and can not be legal. How can they get away with treating customers this way?