local.h in scanf.c

[Doug Evans]

J. Johnston writes:
 > Not true.  An external C header file "has" to be protected.

I left that as a given.  C'mon.

 > So, you 
 > only get it twice if you directly include it twice in the same source file which 
 > is a programming error, however insignificant.

That's a stretch.
Over time who knows how the code might evolve.
The practice of always protecting headers (that aren't intended to
be included multiple times) means one less nuisance to deal with.