Paying for nothing

I was confronted by mondo agent with an mtn sim contract of 8G for R99 every month I took the offer, 2 simcards were delivered to my home I asked of the the 2nd simcard I was told that the system shows 2 sims worth R99 each I told the agent that I want 1 sim not 2 she told me to use only 1 and that the other will be cancelled come monthend I was billed for 2 sims I complained and I was assured it's been sorted and later I was told to pay R140 for the cancellation of the other sim I complained again that how am I liable for the mistake they did the agent said someone will contact me they never did the money was debited from my bank account which I reversed and complained, instead I received a message that my sim will be suspended it's been 2 months now since the sim has been suspended but the bill goes up every month and it's been debited every month though I keep reversing. mtn never attended my query instead suspended the sim but keep debiting pls help