Hi Dashdash,
Thanks for your update.
If the Office 365 users have mailbox (Exchange online license), the best way is to do smtp match.
Here are the detailed steps:
1. Delete the duplicate users in AD.
2. Do smtp match to sync the Office 365 users to AD.
http://support.microsoft.com/kb/2641663/en-us
If the Office 365 users don't have Exchange online license, we have to delete the users in Office 365, sync your AD with Office 365 and then reassign licenses.
If you need further assistance, please feel free to let me know.
Regards,
Jerry
