It's actually a hard problem that people who do this encounter and seems impossible to get right. We actually can tell if a friend is walking in the distance by looking at gait+clothes but could make mistkes as well, it's not 100% accurate. For surveillance reasons it's much easier to get 100 accuracy with just tagging people with microchips and that'a that. Why would we want to excel at gait-fingerprinting people?? That's a scary thing to want to perfect. Very Science fictiony and hopefully not prevalent in the future