On the system you can found them on Configuration>Users

If you are coding:

users_obj=self.pool.get('res.users')
user_ids= users_obj.search(cr, uid, [])
users_obj.browse(cr, uid, user_ids)

The last line it's going to give you all the users as object.

go to Settings>Configuration>Users. and export all the users from there. Get all the ids for each user and as long as you use the ids when representing each user a new user will not be created the system will simply know you are still referring to the same id(s). Each time you add a user the application assumes it is a new one even if the names are the same but if you are adding details using the ids the application knows you are referring to existing names and updates the id/name details rather than create a new one