Unsubscribe question

Hi there -- we are a school in the midst of our admissions process. We are constantly getting new lists of eligible applicants that we need to upload into the Marketo system. If someone has unsubscribed and we upload them a second time into the database, will Marketo recognize that they have unsubscribed or will they continue getting emails from us? We want to make sure we are being as ethical as possible, but are worried that some folks who have previously unsubscribed are getting our emails. Thanks for any insight or help you can provide.