Stanley Paxton and Kinsey Alvianc...
If people only looked at their backs, then people wouldn't be able to see their difference. Apart from the same posture, their height and hair color was also the same.
Stanley was once a Paxton with the signature brown-reddish hair. He changed his surname when someone adopted him when he was in the orphanage.
Even though he is now known as Stanley Calvin, he still had Paxton's trait. No wonder if people would mistake him for Kinsey when they looked at his back.
This time their hair was slightly different because Kinsey's hair looked redder since getting the life energy of the red queen a few years ago.
Therefore, people could still distinguish them even though they only looked at their backside.