It depends a lot on how you are updating.
When you initially choose to sync a device or computer you generally get two choices: Replace or Merge. If you do the first the source replaces everything. If you do the second it will try to combine any contacts that have the (exact) same name. In the case of data that you can have more than one of, like phone numbers, it will simply include all phone numbers in the new contact. In the case of something with only one entry it 'should' take the data from the contact with the most recent edit/creation date. If it can't figure it out, it should leave both contacts untouched, and you will have a duplicate that you must resolve yourself.
Later, when all you are doing is updating contacts the most recent update will triumph, usually. If it does not for some reason, you will be prompted to resolve the conflict. This is common if you are syncing your contacts via iTunes on the computer. I don't recommend this method since it only updates when you do a sync, which could be days or weeks apart.
The better method is to sync your devices and computer through a service like iCloud, Gmail, or even Yahoo. This results in a central database on the server that everything else references. Updates happen frequently, whenever you open your contacts and are connected to the internet, and because of that almost never result in conflicts.
Finally, if you are setting up more than one account with contact data and syncing, the Contact app (both iPad and MacBook) will actually create separate databases for each account. They may look merged if you've selected to view all contacts, but you can also choose to view only the specific contact database you want. You can also link/unlink contacts between the databases to force contacts with the same or different names to be merged/separate in the view all mode.
Hope that helps a bit.
