Handling of duplicate email address
Duplicate email addresses cannot exist in the same list. These are automatically removed from the list when you do an import.
Lists are treated individually, though, so a single subscriber can exist across multiple lists. This allows your subscribers to hold multiple subscriptions - e.g. they may wish to subscribe to 'Brand X' emails sent to a 'Brand X list' but not your 'Brand Y' emails.
For the purposes of billing, subscribers which exist across multiple lists are counted multiple times. We'd often recommend looking at using Tags and Fields to reduce costs.