Double billing on my DSTV account

I recently purchased a DSTV explora and a consultant was sent to install it and disconnected the old decorder to my suprise they diactivated my account stating I was owing them for the other decorder that is no longer in use they are saying the installer should have notified me to call Multichoice and cancel it now I must pay for 2 decorders even though the other one is not in use