Does not respond to queries

I purchased an item on credit from Hi-Fi Corporation in December 2018. My instalment is paid ON TIME every month by debit order, but every month I receive a SMS to inform me that my account is in arrears. In May 2020 the debit order WAS NOT PRESENTED to my bank for payment; I received a phone call to inform me that my account was in arrears. I authorised a special debit order for the next day which was presented and paid as arranged; however, the day after that another debit order was presented and paid AGAIN. Ever since then I have been trying to get an answer out of these people as to WHY they debited my account TWICE in May. Telephone calls go unanswered and e-mails are just responded to with one more meaningless reference number. If I could get hold of anyone I would request the settlement amount for this account and pay it off so that I never have to deal with them again.