Upgrading - Paying for two phones

I upgraded my vodacom contract. At the time the consultant told me that i'l only start paying my new contract amount when my old contract expires. 1 Month later i'm paying my new contract amount and my old phone bill. Iv never been aware of this and MTN never charged these rates. If i was aware of this i would rather wait than to upgrade early. I feel everyone should be made aware of these extra rates before upgrading.