Yeah I understand. If it's PayPal, check your 'active subscriptions' via the website. Maybe it created 2 auto payments resulting in you paying double. If it's already payed I guess the forum/community won't be able to help.
It is indeed rather frustrating to find info about this, the guides don't give a lot of info. When you login and check the Subscription page, there is only 1 active subscription stating Premium or does it still show Family? And is your double payment reflected on the Receipts page?
If you can see it in Receipts on your Spotify profile page it would mean it is still active on their end. If you can't see it in there, I guess the automatic payment was never cancelled and as such the credit card kept paying. In this case ask your CC provider to stop paying the 15 EUR (or USD or...) and only leave the 10 EUR one running, then contact spotify again asking for a refund for the last 9 months showing them the CC receipts.