I was slightly injured in a car accident and treated at Good Samaritan Hospital, Phoenix, Arizona. When I received all the bills, I sent them in to my insurance company.
Which proceeded to pay Hospital of the Good Samaritan, in Pasadena, California.
And insist that since they had made a payment, it was no longer their problem.
And the hospital that had been paid refused to return the money until I could "prove" that I had never been treated there.
I ended up mailing copies of the bills and a notarized statement to the effect of "I swear under penalty of perjury that I was in the emergency room in Phoenix, Arizona, at this date and time and therefore could not have been treated in your emergency room in Pasadena, California, so please return the money" to the other hospital to get them to refund the insurance company, so that the insurance could pay the right hospital.
All told, it took about six months of phone calls, collection calls, collection calls referred to the insurance company, letters, and aggravation.