Being billed for the old handset and new handset after making an upgrade

I have made a cell phone upgrade with Vodacom after twenty months. I am now being billed twice for handsets, the one billing is for the old handset and the other billing is for the new handset. This is very expensive for me, I am told that this will continue for the next four months. I was not made aware of this when doing an upgrade. I have been using Vodacom from 1998, it is the first time this happens to me. I am unhappy and stuck with an extra R507.00 bill for the next four months which I did not budget for. I find this unfair