I would say that a passenger or person booking travel for a passenger may not want to enter "FNU" in any of the name fields for the ticketed passenger's name. I only say that because I've seen ticketed names with "FNU" result in more problems than passengers having their first and/or last names repeated in the first and/or last name fields. Of course anyone with a clue and informed that FNU is for "field not used" or something like that should be able to allow the passenger to proceed as normal, but as with many things YMMV from time to time.