Hey @Emelyy_castillo, and welcome to the Spotify Community!
How long have you been charged twice or is this the first time?
It is possible that you have clicked on the upgrade button more than once (and both payments were credited to your account).
Please contact Spotify Support via any of these links and they'll help sort this out for you.
Contact form: http://bit.ly/SpotifyCF
If you get an automated reply email telling you to check the Knowledgebase or the Spotify Community, you need to reply back to it, even if it's from a no-reply address.