Unauthorized debit

My bank account was debited twice by cell c without even consulting me and when I phoned they told me it was a debit check when I'm already their client and have been paying every month with no problem, if this is the way you treat all your, you really need to rethink and redo customer service. You took my last money for the month, how am I supposed to put petrol, you could have warned me about this debit check and I wonder if this only happens to, I don't know why I'm being picked on like this.