correct a few doubled-word nits in comments and documentation

correct a few doubled-word nits in comments and documentation

Found by running this command:
$ git ls-files -z|xargs -0 perl -0777 -n \
 -e 'while (/\b(then?|[iao]n|i[fst]|but|f?or|at|and|[dt]o)\s+\1\b/gims)' \
 -e '  {' \
 -e '    $n = ($` =~ tr/\n/\n/ + 1);' \
 -e '    ($v = $&) =~ s/\n/\\n/g;' \
 -e '    print "$ARGV:$n:$v\n";' \
 -e '  }'

Why not just git grep -E ...?
That wouldn't work then the doubled words are separated by a newline.
This is derived from a Makefile syntax-check rule in gnulib's maint.mk:
  http://git.sv.gnu.org/cgit/gnulib.git/tree/top/maint.mk

Signed-off-by: Jim Meyering <redacted>
Signed-off-by: Junio C Hamano <redacted>