If I import a subscriber more than once, do they get multiple notifications?

If you are importing or uploading a subscriber into a list they are already a member of, they will NOT be renotified.

If you are importing a subscriber into a different list which they are not currently on, then yes, they will get a notification to advise them of the change.
However, you may not overwrite a subscriber’s unsubscription or removal via import; FeedBlitz will apply this policy to any file or list of addresses you upload.