IDA Law harrasing ne

I am being harrased by IDA Law almost every day about a trauma unit incident that occurred 18 months ago. They keep accusing me of not paying an outstanding account but I keep being provided with a new invoice every time. When will this end. I provided them with a proof of payment and they keep harassing me. Not acknowledging receipt and accusing me of not sending a proof of payment. I am so annoyed and confused. How do I know this is legit.