Unauthorised Debit Order

On 1/2/2016, an additional debit order went through my account from African Bank. Their usual 3 debit orders had already gone through a few days previously without a problem. When I phoned the Client Care Centre, the guy I spoke to (I can't remember who) confirmed that there were no arrears on my account, and will submit it for a refund. When I contacted African Bank today (12/2) as no refund had been received, and was eventually transferred to Collections, who said the additional amount was apparently arrears (which I didn't have on the 1/2, but suddenly did on 12/2). They then refused to refund the unauthorised debit order, over these alleged arrears. This is not acceptable, I did not authorise the extra debit order, nor did I receive the SMS to this affect, that the apparently sent on 12/1.