problem with vodacom

I had a contract phone with Vodacom on August I took another contract phone which subsequently cancel and it was still with in window period, unfortunately they continue to debit money for 2 months I then cancel their debit order but I continue to pay my current contract phone. On December I was credited t he months they debited for 2 months. The very same credit they also took it back they said the contract that I cancel was in arres because I didn't pay it for 2 months. How can they bill me for the contract that was canceled?