Getting Billed Twice for 1 Bundle

On the 29th July 2018 at 18.10 i had R50 airtime transferred to my number. I then tried to purchase 1gb weekly data bundle from the MTN App for R49. I kept getting a error message when trying to purchase the data bundle so i called 135 (contract customer care line) and asked the lady to purchase it for me. Before she purchased it i asked her if the bundle will be paid for by the airtime that is on my phone or if i will be billed at the end of the month for the data bundle.. She specifically told me that the amount for the data bundle will come off of my airtime amount. So i agreed for her to buy it for me. At 18:21 (29 July 2018) i received a SMS saying that a 1gb weekly data bundle has been successfully loaded. I checked my airtime balance and the R50 airtime i had was gone so i was happy that the airtime was used to purchase the bundle.. But when i got my monthly bill i see i got charged R60 for the same 1gb data bundle. So i want to know why am i getting billed twice for the same 1gb data bundle? In total i am paying R110 for 1gb weekly bundle. Please sort this out. Im happy to pay the airtime that was deducted from my airtime balance but im not happy about paying the R60 extra on my monthly bill. Please deduct it off my bill or refund it for my next one.