I often get repeats of the same Contact when I merge or combine address books, so I don't see any restriction on duplicates. I'd set up two different contacts, each with the same address. What's the difference between the two instances of this person? Are they not using their own name in both of the two cases? An email address is unique, but it can have different text labels. (That's how some scammers make it look like they have hijacked your account.)