First of all, did those payments come out of your account on the same day?
If so you probably just hit the upgrade button more than once (and both payments were credited to your account).
You can double check this by going to your Subscription Page and confirming your next payment date is more than 1 month away.
Let me know how you get on!
If you believe you are eligible for a refund, please get in contact with Spotify support and they can take a look backstage. The easiest way to contact them is by messaging SpotifyCares on Facebook or Twitter. They usually respond within 48 hours!
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