MTN 😒🥹

I have a problem with MTN ,I was called by one of the consultants and she was offering me data deals with a new sim card and as we spoke I told her that I don't need the deal coz I already have a contract with MTN that offers me enough airtime to buy data every month. She kept on going with what she was offering and I told her that I cant be adding another contract and she said no this won't be paid or charged for if the sim is not active, okay fine then I was surprised month end MTN debits double the money I pay on a monthly basis, I ignored it due to thinking I was over charged since it was the festive season but I was surprised how can they charge me double without notifying me first. This really makes me angry coz how can a person get charged or pay for something that was said it will be paid for once active, but at the end of the day it was never active or used ? I'd rather be blacklisted than to pay for something that I didn't agree on .