Are you sure that you didn't mistakenly topup/buy membership twice? What does the My History page on your 48 account say about recent payments/purchases/topups? If you definitely didn't inadvertently make a second payment/purchase then you'll need to contact 48 support to get them to sort this out for you.
Hope this helps.
I am not a 48 employee so if you need to contact 48 support then use option 3 or the chat facility here.