Azure users sync question

Hi, i would like to set up user / group sync via Azure application but wanted to ask about existing users.  Will they be duplicated or merged if they already exist.

It seems that there may have been some sync via Active directory quite a while back but there are users that left years ago still listed so will these be removed once the sync is set up or will it need to be removed manually?

I have no problem running some housekeeping on the users/groups once done, just wanted to know the consequences of the new directory services been added.

Hope that makes sense!!

Lee