Duplicate Billing Each Month

Hello, I have a contract which I upgraded on Black Friday in November 2018. Worst mistake of my life. Since that very day I have had billing issue on my account where there is an incorrect payment deducted from my account. (more than double my agreement). I went into a MTN store 3 different times through these last few months to try and fix the duplicate billing. Today, yet again I received another SMS that I will be billed twice again on my account. I even have reference numbers, and no resolution still.

Went to a different branch from where I originally upgraded my account, the MTN system is built so poorly that that person in the store could not resolve my issue or even trace my reference number to try and help me.

I want to cancel my MTN contract ASAP.