Emma dedupes upon import, so even if you have email@example.com in your spreadsheet 10 times, she'll only import the address once.
Additionally, if firstname.lastname@example.org is in your spreadsheet and is also already in the audience group you're importing into, that member won't be added again. Each email address can only exist once in an audience group and once in your overall audience, though it can be in multiple groups.
Despite the address being in multiple groups, if you send an email to multiple groups that has that contact in it, Emma is smart enough to recognize that and will only send the email to that address once.
I hope this helps, but give me a shout if you have any other questions!