Poor customer service in billing department.

Poor customer service in billing department.
I & my wife visited the facility around Sept, 2020. We paid all the bills upfront i.e. right at spot and full amount. One month down the line, I get a medical bill for the charges stating that we have not paid. I provided transaction details (date, ID etc.) from credit card statement and also mailed the copy of my credit card statement. Its been nearly 4 months and their accounts or billing department still sending the same bill.
1. I had overpaid based on the bill generated by front-desk lady. Ideally, I should be getting a refund.
2. I have to follow-up with billing department every 2 weeks. Sometimes they connect me to supervisors too but all in vain. No clue on how to get this fixed.