Upgraded contract

I recently upgraded my telkom data contract. They gave me a new number and sim card. However, I noticed that at the beginning of the new month they added additional data to my old sim card. I called the call centre to find out why. They say that I still have to cancel the old contract and give 21 days notice otherwise they will keep billing me. I dont understand whats the purpose of upgrading and paying an upgrade fee if they dont stop the old cotract. So now I am gonna be paying for 2 contracts instead of one.