Life Healthcare trying to ****** money

I am on Discovery Medical aid. I had an operation in January 2017. Recently I received an account from Life health care stating that I owe over R19000 to them. Can you believe it nearly 3 years ago. To cut a long story short the amount was paid by Discovery a long time ago. A certain Yoland from Life health care even threatened me with legal action. Discovery has sent proof of payment to Yoland but now Yoland will not respond to emails asking for confirmation. I also need an apology. How can this company threaten a person stating that monies are owed which is not the case.