Double Handset Installment for 7 months!

I have had 2 handset installments going off for 7 months now. Firstly, I fully understand that the previous contracts handset installment needed to be paid off, but the contract for my previous phone would most certainly have ended by now. I upgraded the first week of August 2017, Still debiting me for this phone in April 2018 makes no mathematical sense if I am still paying off the old phone? In November 2017, I had logged a call via the call centre about my Handset installment going off twice every month but this call was closed with no response. When I complained about this, I was told that there is no credit due to me. Thats all. It is 4 months later and still I have 2 debits for Handset installments going off every single month. Simply stating you dont owe me anything even though Vodacom is double debiting me for 7 months now is just not an acceptable answer. Please could someone assist.