It could of been due to a system error / glitch / fault on giffgaffs part of things or it might of been a mistake / accident of yours, if you want a refund back, please check out the below. >>
1, Make sure it did take 2 payments or payments you didn’t want to make: https://giffgaff.com/support/questions
● Also : Please be patient, the response may take up to 24 - 48 hours for a agent to reply
Or you could keep it on your account for ongoing or for next month etc.