Incorrectly billed by Lancet

I have been receiving harrasing phone calls from attorneys with rrgards to an outstanding Lancet account. I had bloodtests done without any medical aid and paid for the tests in cash each and every time , I even have the slip to show I have paid upfront.Now Lancet has handed over my details to attorneys claiming I owe them money for bloodtests done with a doctor that I have never been to in my life. I even confirmed with the doctor and he does not have my details on record whatsoever. I have spoken to Lancet before to inform them that it has been done in error and yet they are proceeding with legal action against me. I am fed up!