It sounds like the payment is still going through for premium on Spotify's side even though you now have a subscription through Vodafone.
Check out this support article
I suggest you contact support so they can help you with the refund.
If you get an automated reply email telling you to check the help section or the community, you need to reply back to it, even if it's from a no-reply address.
Support usually replies within 24-48 hours.
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Note: I'm not a Spotify employee.