Deduction on my account without my consent

So I went to Pep home store Tshwane mall on the 19th of December to purchase a cellphone and I was using a card payment, then it happens that the money deducted from my account but it didn't went through to their system, after that they adviced me to go to the bank for reversal, then everything went well and I went back to make a purchase again the amount was R650. Then when I wake up today the same R650 is deducted from my account with the same store, then I went to them the cannot assist me