Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
This forum is public, no one from here has access to your Microsoft Billing account.
Have you already checked whether the duplicate Microsoft 365 subscription appears under https://account.microsoft.com/services?
From https://account.microsoft.com/services find the duplicate subscription, select "Manage," then "Cancel." Microsoft will show during the cancellation process whether that purchase is eligible for a refund.
If you do not see an option to cancel the duplicate subscription, let me know what options you see under "Manage." And let me know where you bought it if directly from Microsoft Store.
Click "Add comment" to let me know.
Thanks!