Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello, good day and thanks for your question. Sorry to know about this and I understand that you were charged twice for Microsoft 365 subscription.
As you have an existing subscription, you will to double check it first via account.microsoft.com/services and check it from there before you cancel anything and then you can go to https://account.microsoft.com/services/microsoft365/billing and try it from there or try to turn off the recurring billing at https://go.microsoft.com/fwlink/?linkid=2277073 and you can also follow the guided steps indicated from the link below which also includes refund eligibility check.
If you will encounter an issue, you can reach out to the support team at https://support.microsoft.com/en-us/contactus/ and then click on Microsoft 365 and then click Get Home Support and then type in Billing question and click the blue button that says Sign in to contact Support and login and proceed from there.