The uncanny double goes way back in storytelling. But every time I see of the stories that has someone...however they achieved their duplicate looks...take over in the life of someone else—that's where my disbelief stops being suspended. Forget magic and glamour and freaky plastic surgery. How do you slip into someone else's life, even someone you are very close to?
No matter how uncomfortable they make the tension on a Ringer, for example—could you take the place of your sibling? If right now, looks didn't matter, could you slip into their social life and their job and their outward behaviours to be able to go more than a day or two? What happens when you need to sign their name? Enter their password? Keep friends and family unsuspicious?
Passwords. Names. Jokes. Bodies of knowledge—do you know any stories that have been even vaguely convincing?