Adsync and automatic user creation upon log in

Right now adsync is not set up.

However, whenever a user (local/domain) is logged into a pc that has endpoint running a user is added to sophos central(no email address though).

If i do ad sync, will there be a duplicate of users (that have been already added) ? Or will the old users simply be updated with the new information from ad sync such as (email, group membership, etc) ?

And lastly, will i be able to use phishing campaigns against these users once i run adsync or will they have to enroll /register into sophos central ? 

Thanks.