Double debit on my account although payment was made

I had a problem with my debits on the 1st April and sorted my accounts out . My debit for MTN wasn’t paid . I paid my account in the 15th April bringing my account up to date . On the 2nd May my accounts was debited for R3636 although I paid my account on the 15th April . I went to MTN Century City in Century Boulevard Cape Town and spoke to a consultant and he lodged a query and said that my money will be returned to me within 48 hrs that was on the 6th May . Up til now I haven’t received my money back . And now I see I’m going to be debited again with the amount of R1823 . MTN owes me R1823 . As my account was up to date on the 1st May where MTN was only supposed to take 1 installment . Yet until now they haven’t paid my installment . Please sort this out as I have all the proof . So according to my knowledge I have a credit of R1823 . So no debit must go off on my account for MTN as they owe me and not me owing them .