User names in multi-tenant application

In a multi-tenant application (multiple customer use the same functionality, but don't share data) the usernames must be unique. So the application manager can get an error: 'a user with this name already exists', however he can not see any user with this name. What is the best way to solve this?
1 answers

Try to recommend a user naming convention to avoid conflicts, such as using email addresses as the user name