Fix Duplicate proxyAddresses Error in Microsoft 365

Go to admin.microsoft.com and log in with your global administrator credentials.

Navigate to Users > Active users. Search for the conflicting email address. If found, select the user, go to 'Manage username and email', and delete the conflicting alias.

Go to Users > Deleted users. If the address belongs to a recently deleted account, you must either permanently delete that account from Microsoft Entra ID or restore it and change its aliases.

Expand Teams & groups in the left navigation menu. Check Active teams and groups as well as Shared mailboxes. Remove the email address if it is tied to any of these objects.

Wait up to 15 minutes for the changes to sync across the directory, then return to the user's profile and try assigning the Microsoft 365 license again.

Open PowerShell as an administrator and run the command 'Connect-ExchangeOnline' to sign in to your tenant.

Run the command: Get-Recipient -ResultSize unlimited | Where-Object {$_.EmailAddresses -match "conflicting@yourdomain.com"}. Replace the email address with the exact one causing the error.

The command output will display the name, recipient type (such as UserMailbox, MailUser, or MailContact), and primary SMTP address of the object holding the alias.

Use the appropriate Set- command (like Set-Mailbox or Set-DistributionGroup) to remove the conflicting SMTP address from the identified object's EmailAddresses array.