Merge pull request #212018 from RobdeJong/patch-162

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

Author: prmerger-automator[bot]

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 onPremisesUserPrincipalName attribute, it is not supported to sync the same UserPrincipalName value to the Azure AD onPremisesUserPrincipalName attribute for multiple different Azure AD users.
 * In the lists above, the object type **User** also applies to the object type **iNetOrgPerson**.
 
 ## Next steps