Collection - How to effectively collect the overpayment from doctor’s office?

I would write a very nice letter explaining how the overpayment occurred and requesting a refund. I would send the letter certified and keep a copy. If a refund is not forthcoming you might check with the insurance company and ask for their help. A further step would be the governor's office of consumer affairs. They may or may not have jurisdiction over doctors. As a last resort you could consider an action in small claims court. Call the small claims court to get information on which county to sue in. If Doctor not incorporated must be in the county he resides in.