My ex-business partner and his son use to do this, as they both had the same first and last names. The son would also use father's FF number on his reservations so that mileage of both of them would go into his father's account. This was OK until one day when the both travelled, and the Airline noticed that there were two flights on same day in totally different parts of the USA which would been impossible for one person to do. They got a slap on the hands from the Airline and were told to discontinue this practice. They did not of course, and were just more careful.