Did you get this error when you were creating a new user? "This email address is already being used as an alias for user <user name>." This article will show you how to remove the email alias from the user.
Tip: Email addresses are how the internet knows where to send your email and all email addresses must be unique - across the entire internet no two email addresses can be the same.
Remove the email address from the user
Before you begin, make sure you've noted which user you need to remove the email address from.
From the Office 365 admin center, you can find the user a couple of different ways:
From the admin center Home page, type the user's name into the search field, and then select them from the list. It will open the user card.
Go to Users, and then select the user from the list of users.
In the Username / Email and Aliases section, select Edit.
In the Aliases section, remove the email address by clicking the remove (wastebin) icon, and then Save.
Note: You can't remove the primary email address. If you need to remove the primary email address, create a second email address for this user and click Set as primary. Then you can remove the user's email address.
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