EXTRA DEDUCTION ON MONTHLY DEBIT ORDER.

My Vodacom started in December 2016. My agreed and signed debit order is a constant of R369.00 on a monthly basis. The contract terminates in December 2018. However after having a look at my December 2017 bank statement, I noticed I had been debited an amount of R838.00. This is a standard contract and the sim card is not in use. therefore there can be no external or extra costs. Is it lawful for a service provider to deduct an unauthorised debit from your account without authorisation. This has now lead to my bank account falling into overdraft and thus costine ME extra bank charges. VERY DISSAPPOINTING AND BAD SERVICE.