"Username already exists"
For example, a deleted user might have been re-created (either by an admin or due to a directory sync application). If you then try to restore the deleted user, you'll see the message. To check for the existing username and remove the conflict:
Search results for workspace unexpected user alias deletion
Troubleshoot Google Workspace Migrate
1. Decide whether to use a user alias domain
Separate a Google Workspace account into two accounts
Add or delete an alternate email address (email alias)
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What's an email alias and how do I create one in Google Workspace? 325K views October 6, 2022Unable to remove email alias with asterisk (*)