Wrongfully Handed over to lawyers;Paid Account

I have paid Avon 3 years ago today I have received a call from their lawyers saying that I haven't pay any cent to them and they need proof that I have paid when I call Avon they say I have paid they will send me the proof that I have paid and this is the second time they have called me this year. I don't understand that I have to be the one fixing this matter while is their matter.