Transportes Mendoza runs two buses on the same departure to Iruya. Why there are not two different departures on different hours, I don't now. But maybe the logistics on the narrow road to Iruya is the reason. Or they want a working bus, if the other bus breaks down. Or it is just a matter of keeping down the staff numbers. To wit, the same guy was selling tickets in both buses. He would ride with one bus, and when he was done with those passengers, he would get off and wait for the other bus.