Billed for old and new upgraded contract in same month

In January( the 4th) I did my online phone contract upgrade. At the end of January (1Feb)I was charged for my old contract. Then again at the end of Feb(1 March) I was charged for my old contract (handset and airitme etc). Then end of March (1 April) I was charged for my new contract for March and charged for my new contract for February. I understand I must still pay the handset fee but I was charged the full month fee (handset and airtime etc) of my old contract for February (1 March payment). So I paid data and airtime for two contracts for February (1 March payment). When I phoned Vodacom the call center lady told me that my double charge was correct and everyone always pays for two contracts for one month when they upgrade. I have never had to pay for two contracts in 1 month before and I have been a loyal vodacom customer for 16 years. Why was I still charged for my old contract in February when my upgrade had gone through and I was loaded with data for my new contract already ?