It looks as though I'm paying for two Microsoft 365 accounts.

Log into any Microsoft account you may have on this link, do you find two separate Microsoft 365 subscriptions

https://account.microsoft.com/services/

If you do find two separate subscriptions, on your PC open Word, then go to File - Account.

On that window you will see the Email address of the Microsoft account you use on the Office apps, do not cancel that subscription, cancel the other subscription using the steps provided on this link.

https://support.microsoft.com/en-us/accounts-billing/subscriptions/cancel-a-microsoft-365-subscription

If you are still unable to find the second subscription, using any Microsoft account you may have, try using the steps they provide on this link to find out what the second payment was for.

https://support.microsoft.com/en-us/account-billing/how-to-investigate-a-billing-charge-from-microsoft-398c5328-364c-d5e4-ea8f-f5ad60562a93