****************! Is this how MTN gets their bonus?

I took out a Top Up contract with MTN that suppose to cost R346.84 per month for 24 months, this is what i signed for, this month with no prior warning they just debited R691.54 and when i contacted the Call Center they say i registered for extra data which i used between 11 Oct and 11 Nov, but they cannot tell me what i used it on. This is outrageous as i have uncapped WiFi at work which it what i use from 8-5 daily and on weekends i used WiFi at home, i rarely ever finish the 2gigs of data i receive as part of my package and on the occasions that i have, i have had to load data manually so where does this come from??? I feel like this is ******* and i would not recommend anyone take out a contact with this company as being ****med for any amount of money is unfair. I am not able to get a proper response from the MTN staff and have just been told i need to pay it and if i want to cancel my contract i will be charged a fee. If i decided not to pay for a month or 2 they would blacklist me, yet they can double charge for no reason and i must accept this! its pathetic.