I was charged twice

If you're sure that that "double" charge comes from Apple (check your bank statement very carefully, ignoring scam messages that might be addressed to your iPhone), start reading this support page:--> Subscriptions and Billing – Official Apple Support, and check each of the first 3-4 sections.

If you cannot find the information you were looking for, please contact iTunes Store Support Team directly and explain your problem, using this specific link:--> Apple Support

(if you get no help, look at the bottom of the link, where you can contact them using "chat" or "phone" options).

Regards

Giulio

Maybe you have subscribed using two different Apple IDs