Double payment taken but no refund offered

I am absolutely disgusted by the service that I received from Dischem in Canal Walk. I visited the store for medication on 22 December 2019. I paid using my bank card and 3 times the cashier informed me that the transaction was declined. Nothing to this effect appeared on the machine on my side and no slip printed out. After the third time I made the payment in cash. The next day I saw that all 3 attempts (which were supposedly declined) came off my bank account, so that means I paid for the medication 4 times. My bank corrected the account the next day and refunded two of the payments to me. When I queried with dischem that they had received an electronic payment and a cash payment for the same medication, they told me the onus was on me to provide them with proof, as the cashier's till had balanced, which does not make sense at all. when I provided my statement from the bank and my receipt of the cash payment, I was informed that the electronic payment did not reflect on their side at all, which is ridiculous as it was definitely deducted from my bank account. So this statement absolves them completely and I paid double for my medication.