payment allocation

I received an invoice with a link to make payment, I paid on the very same day. Days after the payment, I received invoice from them demanding the very same payment I have made. I called their accounts department and they requested that I email them the last 4 digits of the card number I used to make the payment and I did that but I keep on receiving emails from them demanding their payment. I keep calling them and each time I will be asked the same information that I have send and will be told that they are escalating my case to their finance section because they are the ones who have to allocate my payment or they will transfer me to them and their number will never be answered. So I just wonder after how many phone calls will I have my payment allocated or maybe their employees are having debt recovering companies, by not allocating our payments, we will eventually be handed over so that their businesses can benefit. It is a real nightmare dealing with ER consulting. I even wonder if their finance section really exists. Their service is very unprofessional.