Again?

Ugh. So, my old dentist has been claiming that I didn't completely pay a bill. They are claiming I only paid for half of something. Except I did completely pay it. A few months ago I called, and I thought we'd worked it out. They finally agreed I'd payed. And then I didn't hear anything for months.

Except I just got a collections letter. And I called again and they are again claiming I didn't pay and need to pay.

I don't even know what I can do at this point. I sent them a copy of all my payments (I paid for a whole bunch of things together up front, and they didn't give me an itemized receipt, only one showing my full payment). They are still insisting I owe them a couple hundred dollars I don't have.