Hi Juli, Marketo will automatically overwrite information based on email addresses -- assuming that the fields aren't blocked from list imports. If there are new email addresses, then new records will be created. Any email addresses not on your list won't be updated.
You will be able to tell at the import stage if the fields are blocked from list imports. To change these preferences (something that you'd probably just want to do temporarily), go into the Admin section, and change the "block field updates from" on each field that you need to be able to update via the list import.