Refund denied

Hey on 21 December 2020 I made a purchase at mrp Tygervalley my slip was emailed to me only for me to see recently that I was billed twice for the same item I made contact with the contact centre only for them to tell me that I need to return all the items and I need to proof to them that they billed me double. How isit my responsibility to proof to them that I was billed twice? I need mrp to refund my money. I understand human errors occur as its December the stores are full so why is it now my responsibility to proof a reasonable doubt.