Two account with same UPN in Active Directory

Question

Is it possible to have two accounts with same UPN in AD? 
I'm using Windows Server 2012 R2

Answer 1

To expand on what Mr X said. Active Directory allows you to assign just about anything to the userPrincipalName attribute, as long as the value is unique. The value of this attribute is generally referred to as the UPN. But you could assign values with no
"@" symbol, for example, or more than one "@" symbol. Also, Active Directory does not require that any value be assigned. It is an optional attribute.
If no value is assigned to the userPrincipalName attribute, then the user can use a "default" UPN, which is <sAMAccountName>@<DNS domain name>, where <sAMAccountName> is the "pre-Windows 2000 logon" name of the user
(the value of the sAMAccountName attribute), and <DNS domain name> is the DNS name of the domain.
If a user has a value assigned to userPrincipalName that does not match this "default" form, then they can logon with either UPN, as long as no one else has either value assigned to their userPrincipalName attribute. You will note that Active
Directory enforces uniqueness in the domain on sAMAccountName values.
If in domain "domain.com" a user has a sAMAccountName of "jsmith" but userPrincipalName "[email protected]", then another user can have userPrincipalName of "[email protected]" (as long as no one else
has that value assigned to userPrincipalName). They can each logon with their assigned values for userPrincipalName. But the first user can no longer also logon with their "default" value, based on their sAMAccountName, because it is now
used by someone else. But if you attempt to assign a value to userPrincipalName that is also assigned to another user, you get an error.
Bottom line, only one person can logon with any given UPN.
Richard Mueller - MVP Directory Services