Thanks for reaching out about this here in the Community.
Hope you don't mind me replying in English.
It'd be good for you to take a look at this support site article and follow the steps under More Help > Do you have more than one Premium account? to see if you can cancel the unwanted "extra" subscription.
Hope you'll find this useful. Let us know if there's anything else we can help with!
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"There's more to living than only surviving." - Dexter Holland
I will answer in English as my French isn't that good, but I hope you can still understand me:
Can you maybe explain how this happened? Usually, when you change your Spotify plan from standard premium to duo, you automatically won't pay the standard anymore. Did you create a new account, or can you maybe be a little more specific? Even if you and the other person had the normal Spotify premium, you will only have to pay for the Spotify duo. Are you sure that you paid double?
I hope you can get back to me soon so we can solve this, and I wish you a great day.