All their lives, my 20-year-old nieces have been pestered by the question "What's it like to be a twin?" Which of course is impossible to answer because they don't know what it's like NOT to be a twin.
And obviously, what people are ineptly trying to say is, "WOW! How does it feel to NOT have the existential loneliness that you were born alone, will die alone, and inbetween no one will ever understand you?!"
Totally appropriate smalltalk.