Debited my Account After I Cancelled It

First I emailed mtn telling then that I want to change my debit order date from End of the month to the 15th of every month, because my pay date changed to the 15th of every month. I did that with all other debit orders in my name and I didn't have any problems except of cause for MTN who failed to debit their money on the agreed date. I then had to follow up with them as to why they have taken their money yet, which they told me my next debit order will be on 15th of October and explained my account is current. Then came 15 October, MTN didn't debit their money as promised. 8 days later I then emailed then again asking why they didn't take their money as promised 2 days later a Sifiso Maoka told me they tried to debit but the debit order did not go through which doesn't make sense cause I had just got paid on the 15th, and even if it wasn't there it would have gone through and reflected a negative in my account. I then cancelled the debit order and got a confirmation email that my payment method has been changed I then asked for MTN's banking details and paid the account myself. Suprising enough a day later I get an notification that MTN debited. Why? After I just paid you!