Double deduction of installment on an account that's not in arrears

Good day, my name is Nwabisa Mngoma and I have a home loan with nedbank. The reason i am deciding to write is because its not the first time i am experiencing a challenge with them especially when it comes to them deducting their installment. Today i realise they double deducted on my account which i didn't make any arrangements of such nature and my home loan account is in advance and not behind with any installment. I called to enquire for the reason of double deducting especially when the is a debit mandate that i sign which is suppose to prevent such from happening. I didn't get any reason except being transferred to the refund department which stated that the refund will take 24 to 48 hours. Why is that especially if i am banking on nedbank on a problem coursed by them. I am really not happy at all