double deduction

After an online purchase using an OTP the same amount was deducted off my account, 3days later, without any authorisation. it is worrying that they have the ability to debit accounts without authorisation. I have contacted them they have been unhelpful passing blame to the bank. I have lodged the a dispute with my bank. its has been 4 weeks and this is unresolved. I think this is extremely unfair towards customers as this becomes my problem to solve whilst I didn't authorise the second transaction and my duty to trace down my money.