Handed over to debt collection for invoice under someone else's name

I started receiving calls from Pathcare regarding an overdue account for someone named "E Nissen", after which I informed them that I did not know this person. I also started receiving snail mail for this person, but did not open the mail as it was not addressed to me. I have now been informed that I was listed as the patient on E Nissen's account, and that I have been handed over to a debt collection agency for outstanding costs. Two calls to their call center was unable to resolve the problem, even after I informed them I am willing to pay my full bill without debt collection costs. How was I supposed to know that I am liable for the costs of the account is under someone I have never known's name, and I only found out it was meant for me after being handed over to collections?