Mtn double debit

I am a customer with 3 contracts at mtn for many years. My monthly payment is R527 and suddenly this month on the 7th a doubke debit went off. I phoned mtn and asked for a explanation twice Tuesday the 8th. Thet could not give me a better answer than my account is in arrears. I told them I want a prper expjanation. I was told that her manager will contact me which never happened. I went into the store in Strand. I was tokd I must bring bankstatements with proof oc oayment which is rediculous as she could not tell me when I did not make a payment so which bankstatement and fir hiw long am I suppose to give proof. She also told me she cant send a request without bankstatements. I then realised that mtn cganged systems and now want payment in advance without notifying a customer but she also stilm insisted on bankstatement which I dont know fir what reason if It changed to a new system and I am not in arrears. How can a company decide to take double debit and not inform a customer and this December after you got a contract with them to pay the way you are paying