billled twice for december 2014

Good Day. Nashua mobile handed cell c contracts over to autopage Nove 2014. I have an invoice from Nashua mobile and was debited for December 2014 and so did Autopage(whic is correct) Nashua sold the debt to creditworx and now Creditworx claiming monies for December 2014. I have tried to explain this numorous times and its not yet resolved. In the meantime my account is acumilating interst. The person at credit worx tells me this invoicing dated 01/11/14 is for October and invoicing 1/12/2014 is for November. BUT A LOAD OF RUBISH -- all contracts are billed in advance. i want this sorted immediately. I have all the invoices to state my point