Attribute Uniqueness in Azure Active Directory

As I dive deeper into Azure Active Directory, I am learning quickly that AAD is a very different animal than on-premises Active Directory Domain Services (AD DS). While both solutions provide identity, authentication, and authorization services, they do so in very different ways. These differences require organizations to be prepared to adjust standard processes to get the two services to work together. Today I will focus on the identity portion of the solution and how the different attribute uniqueness requirements in AAD and AD DS can introduce the need for evolution of management processes for AD DS.

The attributes I want to focus on are userPrincipalName, proxyAddresses, and mail. In AD DS userPrincipalName is a single valued attribute, proxyAddresses is a multivalued attribute, and the values included in those attributes must be unique to the object in the forest. The mail attribute (the attribute that populates the E-mail field on the General tab of Active Directory Users and Computers (ADUC)) is a single valued attribute that doesn’t have a uniqueness requirement. In AAD all three attributes retain their single value or multivalued properties, however, the uniqueness requirements change considerably.

AD DS allows these values to be duplicated across different attributes. For example, one object could have a userPrincipalName of and another object could have a value in its proxyAddresses attribute of The same goes for an object that has a mail attribute of and another object has a value in its proxyAddresses of

In AAD this is no longer true. User, group, and contact objects synchronized to AAD from AD DS require the userPrincipalName, proxyAddresses, and mail (also targetAddresses if you’re using it) to be unique among all objects in the directory. This means that each of the scenarios I discussed above will create synchronization errors. You can’t have one user object with a value in the proxyAddresses of and another use object with mail attribute of

What happens if you do? Well, let’s make it happen. In this scenario we have two user objects with the configuration below:

Object 1
Sync Status: Already synced to Azure AD

Object 2
Sync Status: Not yet synced to Azure AD

After we force a delta synchronization of Azure AD Sync, the errors provided below pop up in Synchronization Manager and an email alert:



The net result of the above won’t synchronize correctly to AAD and the user will be unable to authenticate to AAD. How about two user objects with the same mail attribute? That’s a common use case, right? Nope, same issue. Take note that just because you receive an error saying the issue is with a duplicate value in the proxyaddresses attribute, it could be the userPrincipalName, mail, or targetAddress of another object in AD DS.

Small differences like this can lead to major changes in how organizations manage AD DS when they begin their journey into AAD. The key take away here is to understand that AD DS and AAD are not the same thing, the differences need to be understood, and you must be prepared to evolve existing processes if you wish to leverage the solution.

I’ll end this with a thank you to Jimmie Lightner from MS for his blog post that brought light to this issue many months ago. You can read that post here.

P.S. Take note that if you opt to an alternate login ID (separate attribute from userPrincipalName for user identifier in AAD), the uniqueness will carry over to that attribute as well.

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