Daylight ******s

I was contacted by mtn to upgrade my contract. I did so over the phone on the 9th of September 2024. Thry decided to bill my existing contract for September for the full month as well as prorata the full month for the new contract. How can you charge me for 2 contracts simultaneously. You supposed to cancel one then start the other. I have been with mtn on data contracts for over 6 years. No assistance over the phone either