debt collectors nightmares

I took my child to Chris Barnard Hospital in October 2014 - and then paid my account through a Nedbank - I received confirmation from the hospital that the account is settled - not Vanessa Koen is telling me that although I have paid the hospital , a Dr G Visser would have to be a separate bill I would need to pay - I have never seen a Dr G Visser although this name was on the hospital statement I received - I have sent the proof I received from the hospital to Vanessa Koen - - why now should I have to go through all of this just because there is a mix up somewhere in the pipeline - I have since mailed the contact person at the hospital - a consultant at Dr G Vissers practice and Vanessa Koen - im at my ropes end trying to sort out an issue I have already settled -