Hi Mossiak! Sorry to hear that that happened. When you previously canceled Spotify, did you receive a confirmation email confirming the cancellation? If not, then unfortunately it's likely that the cancellation process was never completed (there are about four or so steps). In this instance you'll want to attempt canceling your subscription again. Again, make sure you go all the way through the process until you get a confirmation email.
If you did previously get that confirmation email and are still being charged, then it's possible that you have a second account that managed to subscribe to premium, which does happen from time to time. To remedy this, I would recommend getting in touch with support. This can be done by either reaching out to@SpotifyCares on twitter; messaging support on Facebook; or through this contact form. If it's been more than 48 hours without a response, reply to the e-mail you received confirming they got your message (if you used the contact form), or, if you messaged them through Twitter or Facebook, shoot them another message there.
As far as a refund, I cannot make any promises or guarantees, but your best bet at getting one will be through one of the above means of contacting support. Again, I cannot guarantee that they will grant you one, but they do do their best to remedy whatever problems they're presented with.
Best of luck, and please don't hesitate to reach out if you have any other questions I can help answer. Cheers!