Fixing the “A user with this name already exists. Use a different name.” Error in Microsoft 365

When managing users in Microsoft 365, you may encounter the following error message in the Microsoft 365 portal:

“A user with this name already exists. Use a different name.”
“This email address is already in use by an active USER”

This issue typically occurs when you try to reuse an old username (that was previously renamed) or attempt to assign the same email address as an alias to another user. Even if the account was renamed or deleted, the email address might still be associated with an old object in Microsoft 365.

Problem

In Microsoft’s documentation (https://learn.microsoft.com/en-us/troubleshoot/microsoft-365/admin/authentication/office-365-user-with-name-already-exists), the suggested solution is to use PowerShell commands such as Get-Mailbox to locate and remove the conflicting object.

However, in some cases, the mailbox does not appear in the Get-Mailbox results, even though the error persists. This indicates that the email address is still tied to the old user account as an alias, even if the account doesn’t currently have an active mailbox.

Solution

Here’s what worked to resolve the issue:

1. Go to the Microsoft 365 Admin Center

   • Sign in at admin.microsoft.com

2. Navigate to Domains

   • Go to Settings > Domains or directly open: https://admin.microsoft.com/#/Domain

   • Select the domain associated with the problematic email address.

3. Check the Users tab

   • Under the Users tab of the domain, search for the problematic email address.

   • You may find that the email address is still assigned as an Alias to a previous account (check the Aliases column).

4. Assign a temporary license

   • If the account doesn’t have a valid Microsoft 365 license, you won’t be able to edit aliases.

   • To fix this, go to Active Users and assign a license temporarily (you can use any available license or borrow one from another user).

5. Remove the alias

   • Return to the domain’s Users tab and edit the account.

   • Remove the old alias that is blocking reuse of the email address.

   • Save the changes.

6. Revert the license

   • After making the change, you can safely remove the temporary license you assigned.

Result

Once the alias is removed, you will be able to reuse the email address for:

• Creating a new user with the same name, or

• Assigning it as an alias to another user.

Even if the Microsoft documentation doesn’t reveal the conflicting object via PowerShell, check the Domains > Users section in the Microsoft 365 Admin Center. Old aliases tied to unlicensed accounts are often the culprit.