Don't believe your eyes

There was a Book Fair at school today. Jack & Molly's classes were both milling around the Hall, and among the adults were my sister and myself. I was on a teeny bench, reading to Moll + Onlookers, and Tara was catching up with some of the other parents.

One of Jack's friends ran up to our little reading group and stopped dead, staring at me.
"There's two of you!"
He looked behind him - to my sister - then turned back to me.
"Which one is the real Tara?"