There is a number of errors like below shown in CCP > More Services > Directory Synchronization > Sessions:
ID 40235 Status Error Message User 'XXXXXX' failed with error: 'User with login 'XXXXXX@domain.local' already exists.', 1103 Directory Name CN=XXXXXX,CN=Users,DC=domain,DC=local
The user has already been created in the Provider's AD.
To make OA ignore this and just overwrite the user's data from the Customer Active Directory, in CCP > Directory Synchronization > Service > Service Configuration tab temporarily change
Conflict Resolution Policy from
Resolve manually to
Overwrite (Customer's data overwrites corresponding Service User). The next ADSync session will successfully synchronize such users.