Enrolled User unable to login to Secure Hub, while 2 users in Active Directory have same UPN(UserPrincipalName).
The Active Directory Users and Computers MMC (ADUC) will enforce unique values, However, duplicate values for userPrincipalName can be assigned to the users via code.
Consider In AD that 2 users namely "Example A Test" and "Example B Test" have the same UPN = example.test@abc.com
In this case - We need to make change UPN and retain the Uniqueness of UPN values in Active Directory for the user to login.