Vodacom incompetency costs their clients money

I have a standing debit order with Vodacom but i choose to pay via EFT well before the 01st of the month, as I typically can receive my salary from the 22nd. As per usual I was paid on the 23rd November, so made my usual payment to Vodacom, only this month as I had not received my invoice, I made a payment based on my average value over the year. When my bill arrived on 28th November, I calculated the difference and made a manual EFT payment to ensure I have caught up. I then noticed on Saturday the 02nd December, that Vodacom had debited my bank with the amount I had already paid, so i asked the bank to reverse the debit order. I am now receiving text msg from Vodacom stating that my debit was returned unpaid, implying that i had insufficient funds ****rue, it was a reversal). when I called vodacom now, not only did the customer service rep seem unable to understand that I cannot be held liable for an amount I had already manually paid BEFORE the debit order, but I absolutely refuse to pay a further R100 for a unpaid Debit return - this is on Vodacom, if your bills are late, clients are impacted. I made manual payments which there is an extensive history of me doing. I have every right to reverse an unsolicited and unagreed debit, and am not liable for the reversal of debit, as the amount was paid in full by me BEFORE the debit. 20 years, and I am seeing a steady decline in customer service. Quick to take our money, over charge us and cut us off, not so quick to be client friendly. I am seriously disappointed in the continued poor service.