Error - AttributeValueMustBeUnique in Azure AD connect sync

My customer had already created accounts in office 365 and managing them in Azure, however due to some changes in business they wanted to sync AD with Azure to sync Password and Manage Identity form AD.

Solution - Deploy Azure AD connect on ADDC, and post that it will do a Soft Match.

However there were error with some users, their identities did not sync and their status still reflected as Azure AD.

Error - 

Unable to update this object because the following attributes associated with this object have values that may already be associated with another object in your local directory services: [ProxyAddresses SMTP:user@domain.com;].  Correct or remove the duplicate values in your local directory.  Please refer to http://support.microsoft.com/kb/2647098 for more information on identifying objects with duplicate attribute values.

Tracking Id: b8367c95-ae67-46e1-xxxx-xxxxxxxxxx
ExtraErrorDetails:
[{"Key":"ObjectId","Value":["cd088468-bb6a-40f9-xxxx-xxxxxxxxxx"]},{"Key":"ObjectIdInConflict","Value":["d7e8405c-54d5-41c6-xxxx-xxxxxxxxxxx"]},{"Key":"AttributeConflictName","Value":["ProxyAddresses"]},{"Key":"AttributeConflictValues","Value":["SMTP:user@domain.com"]}]

I tried to do a Hard match following the article - https://blogs.technet.microsoft.com/praveenkumar/2014/04/11/how-to-do-hard-match-in-dirsync/ 

This did not helped 

From the Azure AD connect health wizard took Object ID of On-Prim AD  and tried to Map it against the Azure user 

Set-MsolUser -UserPrincipalName user@domain.com -objectId "1xxxxxxxxxxxxxxxxx=="

Ran the Azure AD connect sync - Did not helped

From the same place (AAD connect health page took Source Anchor

of AD object and Tried to map it 

Set-MsolUser -UserPrincipalName user@domain.com -ImmutableId "1xxxxxxxxxxxxxxxxx=="

This time it was successful