Hey @Nazzz ,
Thanks for reaching out.
Unfortunately, you'll need a recurring payment method to upgrade to Premium Duo.
You might be able to redeem the Gift Cards for Premium and then upgrade to Duo so that the existing Premium time will be converted into Duo time. You'll still need a recurring payment method for that upgrade, though.
Let me know if you have any other questions.
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Note: I'm not a Spotify employee.