Hi, and welcome to the Apple (user-to-user) Community
Re: "What should do I do? Who should I contact?"
Wondering where the double charge purchase was made from:
eg An Apple purchase using your Apple ID or a purchase from a merchant or website.
If the purchase was for items or services from Apple:
To check to see just what you were double-charged for, if you want to:
If you see 'apple.com/bill' on your billing statement - Apple Support
Three options:
(1) You could try reporting a problem:
Report a problem:
https://reportaproblem.apple.com/?s=6
or
(2) you could just apply for a refund, and wait to see what Apple decides:
Request a refund for apps or content that you bought from Apple – Apple Support
or
(3) You could reach out to Billing and Subscriptions:
There is a “Get Support” Link (or an App) at the bottom of their webpage (link below).
Subscriptions and Billing - Official Apple Support
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Else:
If the double-charge was for a purchase from a merchant / website using Apple Pay:
Check to see there are still double charges, (and that one has not been refunded):
See your Apple Pay transaction history - Apple Support
(NB: For the most accurate record of your transactions, refer to the credit or debit statement(s) from your card issuer ... or was double actually deducted by your bank.)
Two choices:
(1) How to: Get a refund for purchases made with credit or debit cards using Apple Pay - Apple Support
or
(2) Apple Pay Support:
Click on "Get support" (eg: online chat, phone, eMail etc.) at the bottom of webpage:
Apple Pay - Official Apple Support
All the best :-)