It is because the user uses the same email address registered a personal Google account before. For now the same username becomes a Google Apps account, the user need to remove or rename the old personal account.
The next time that user signs in to a consumer product, they'll be asked to change their account. For example, if firstname.lastname@example.org has a conflicting account with your organization, Jane would see these options when she signs into a consumer product:
- Change to an account with a Gmail address (email@example.com or another available Gmail name).
- Change to an account with a different email address (uses her firstname.lastname@example.org address).
Sign in with a temporary username Google provides (email@example.com).