PEP made a mistake nowvthey hold me responsible

Good day. I have an account at PEP /Ackermans. One month I couldn't go to the store to make a payment so I paid via eft with the refrence number they sent me. Now they can't find the payment and they keep on calling and sending sms to tell me I must do a double payment. I sent them a proof of payment and still I must wait 7-10 working days. I refuse to do any payment until I get a answer from them. I feel this is not my fault.