Update reference-connect-sync-attributes-synchronized.md

Author: RobdeJong

articles/active-directory/hybrid/reference-connect-sync-attributes-synchronized.md

@@ -461,6 +461,7 @@ Device objects are created in Active Directory. These objects can be devices joi
 
 ## Notes
 * When using an Alternate ID, the on-premises attribute userPrincipalName is synchronized with the Azure AD attribute onPremisesUserPrincipalName. The Alternate ID attribute, for example mail, is synchronized with the Azure AD attribute userPrincipalName.
+* Although there is no enforcement of uniqueness on the Azure AD onPremisesUserPrincipalNameattribute, it is not supported to sync the same UserPrincipalName value for multiple different Azure AD users.
 * In the lists above, the object type **User** also applies to the object type **iNetOrgPerson**.
 
 ## Next steps