Short answer
First check whether the charges are two different plans, a renewal plus a new purchase, or a true duplicate charge.
What to check
Compare the dates and amounts.
Check whether one charge is a renewal and the other is a new purchase.
Open the app and review the plans on the account.
If it still looks wrong
Contact support and send:
screenshots of both charges;
receipt or invoice numbers if available;
purchase email;
charge dates and amounts.
Support will review the case.