FNB Reversal pay out

I bought 6 gig MTN Data using my FNB App. The first time I got an error massage indication that the transection did not go through. I subsequently tried again and it went through. I later realised that I was double charged and hence I was double credited making my data balance 12 gig. This is a monthly data so I want one of them to be reversed back into my personal FNB account. I went to the FNB Boulders branch and I was told that there is nothing they can do I should go to MTN. When I got to MTN they told me there was nothing they can do I must go to FNB. I have the names of the consultants I dealt with. I don’t understand how is there no process or provision to deal with this. I really need the money to pay for my transport fees….Can someone please help??? All this happend today 5th March 2020 my MTN number is *** and my FNB account is *** the transactions where done on my credit account.