how does syncing contacts in Windows Live Mail work?

1. I have contacts in my Windows Live Mail program on my laptop that are different than the contact list on the people.live people list.  I think this is because I did not sign in to Windows Live when I used the Windows Live program on my laptop.  If I now sign in when I start Windows Live, will the contacts on my computer erase or override the contacts online or the otehr way aroung do do they get combined?  The online contact list is the list I would like to use because it has some names that are not on the list on my computer.

2.  The online contact list has many duplicate names.  What causes names to duplicate?

thanks,