Billed twice for one order

I placed an order in July 2015 during the middle of the month and my invoice number was 42108 and the billing was for R733.55 and it was delivered and I paid R 700 but to my surprise the statement came back stating that I still owe justine R770 from invoice *** that was placed in july and when i queried it they said that i had placed the order, and i then did my own investigation only for me to realise that i have been billed twice for the same order but the billing was different . I explaned to them and they said they will do an investigation and we are now in the october month the issue is still not resolved. This has been done to me before i once paid R990 and they said they couldnt track it even though i sent the proof of payment with the right reference number (it took them 2 months to allocate the money. am tired of sending emails and calling coz nothing gets to be done all i want is for them to sort out my billing because this can put a bad impact on me.