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First of all, did you sign up for a 7 day trial? If so you wouldn't have had to put in any card details. Your subscription will expire in a few days and you'll revert to a Free account (with no charge).
If however you remember putting your credit card details into the Spotify website, then you may have two accounts. Check your email inboxes for a confirmation email from Spotify to determine which one's linked to your Premium account.
Finally, how much are you charged each month? If you signed up via iTunes by paying within the app you will be charged 12.99 rather than 9.99. Please cancel that payment via Apple.
If you're having trouble finding your Premium account just reach out to the team via this contact form.
Let us know if we can help with anything else, have an awesome day!