How exactly does a passenger get on the wrong flight?

LOS ANGELES — How exactly does a passenger get on the wrong flight and go unnoticed for hours?
That’s the question that Chrissy Teigen and many others are asking in the wake of a passenger mix-up that caused a Tokyo-bound flight to return to Los Angeles after eight hours in the air.
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Two brothers were both going to Tokyo but were booked on separate flights, two law enforcement sources told CNN. One law enforcement source told CNN the two brothers have very similar names. S