I have been billed twice.

To check whether this was a true duplicate charge and request a refund:

1. Sign in to your Microsoft account order history and look for two purchases of the same amount close together. If a monthly or recurring payment was previously delayed because of insufficient funds, a late payment and the next on-time payment can appear close together without being a duplicate charge.
2. If only one charge appears in order history, go to Payment options / Manage your payments and select Investigate. This can help find charges from another Microsoft account, such as a family member using the same card.
3. If the charge is for a Microsoft 365 subscription and a refund may apply, cancel the subscription first and then check refund eligibility.
4. To speak with support, use Contact Microsoft Support. Enter the billing problem, select Get Help, and then Contact Support to be routed to the best support option.

Important details:

• If the subscription was cancelled too close to the billing date, a payment may still have been taken. Cancellations must be more than 2 days before the billing date.
• If the subscription was purchased through Apple App Store or Google Play, cancellation and refund must be handled through Apple or Google support.

References:

• I've been charged twice
• How to investigate a billing charge from Microsoft
• How to get a refund on a Microsoft subscription
• Unable to cancel your Microsoft subscription
• Microsoft subscription refund policy