So, your contact is a member of two different subscriber lists. When you send your campaign, you select both of those lists to send the mailing to. Your contact will not receive two emails. The contact will only be sent to once. We know that that contact is not really duplicated, it's just a member of more than one list.
If you are emailing a campaign multiple times, you do have to be careful. If you send to the first list your contact is a member of, and then send again to the second list, that contact will receive each mailing individually. This is because you've sent an entirely different mailing.
But even when sending multiple times, you can protect yourself from duplicates! Just use the Don't Resend option when you're doing multiple mailings of the same campaign! This will ensure that anyone you've already sent this campaign to will not receive it again.