Just move debit order date

How hard can it be for a business to change a debit order date? I asked Sanlam to move my debit order from the original date to the first of the month. They told me that if I do that, I must pay them double. Whenever I ask, they always say the debit order is already being processed. After paying them at the beginning of this month, they still expect me to pay double at the beginning of February, according to their statement. This is absurd!