Debt Collection issue I need help with.?

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I have had this company calling me 3-4 times a day as an attempt to collect an already paid debt. Some background here I have no outstanding debt all my debt is current and in good standing. This collection agency must have bought an old debt and are now pursuing it. The only issue is the debt was paid in one payment in full and I have the receipts to prove it. It was an old hospital bill were the procedure was covered but the anesthesia was not so I paid for it in full as soon as the bill came in. I paid for it by check and still have the receipt and a copy of the check from my bank in my bank statements. I have told them this multiple times and they get very angry. I have also called the hospital and they have on record the bill being paid just as I do and I collected the contact information for the hospital to give to the agent so that he can see for himself that the bill is no longer owed and was never defaulted. He refused to listen instead he cut me a deal to pay only half of the bill and the collection fees. I informed him in no way would I pay a debt I have already paid even half with interest. He became very hostile. I asked for the company name and address and I would gladly send a copy of my receipt and contact information to the hospitals billing department so that they can clear things up amongst themselves. He also did not want to hear that. I know there is nothing he can do but possibly put this on my credit which I will immediately challenge. That is just the issue I do not want it on my credit at all. I have great credit and the calls are getting rather annoying having to silence my phone several times a day. What can I do to get this person to stop?

Debt Collection issue I need help with.?