US Anaesthesia Partners (Denver & Toledo) Doubling the bill !

I had cataract surgery. I received a bill. I called to make arrangements on my bill (two months and complete payment).They agreed to the two month payments. I submitted the first payment early. I submitted the second payment early. I paid the complete bill with the second payment. Subsequently they assert: "we have the check you sent", "we never received a check", "I'll put the second check payment in my notes for other people not to call you about the bill". "we don't make notes on the accounts", "hey, give us your credit card number and forget about us sending you your second check", "really there is nothing we can do to help you so we will be calling you daily until you have paid us", "sorry, that's just the way it  goes, we can't straighten it out". Nor were they interested in checking to see the payment sent !