Look, it was '60s TV. They were on a budget, and they probably just dropped the same extras into the background of the bar scene and hoped nobody would notice. Screens were smaller back then, pictures were fuzzier, and they didn't have HD home video and Internet screencap archives to allow fans to do facial-recognition searches for background extras. Viewers were supposed to pay attention to the people who were in the foreground and actually had lines of dialogue. It wasn't unprecedented for the same extra to show up in two different roles in the same episode (Eddie Paskey did so on a few occasions; for instance, in "City on the Edge of Forever," he appeared as Lt. Leslie as well as the driver of the truck that killed Edith.)
Obviously some sort of "12 Monkeys-like insurance".