Duplicate billing

I had an existing 1 contract with Telkom which i was paying R230 a month and i just opened up another contract in May and debit order just went like R3250 in June. when i ask what is happening they cant even explain to me and they telling this is confusing for them. They created duplicate accounts so im getting double billed. In July the debit order was R5020 just for 2 contracts. and when i ask they keep saying they dont know. This is like car installment. Now they want to charge me even more for their error and affect my credit score because of their incompetency. How is it for a big company like Telkom to have staff that dont understand their own systems and billing, now i we have to suffer because they are not trained well.