DOUBLE DEBIT ON POLICY PREMIUM WHEN IS UP TO DATE.

I have a policy with Absa bank . My premium debit order was on the 26th of every month. In march 2019 I changed the debit order date from the 26th to 21st of every month. They then debit my account with 2 premiums in March 2019 and they said it was for March and April. When i contacted customer service and I was advised that they took 2 premiums because I changed the debit order date of which it does not make sense as my policy was never in arrears. Furthermore I was told that how their system works.