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 <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gitweb/static/js/lib/cookies.js

@@ -30,7 +30,7 @@
  *                            If a negative value is specified or a date in the past),
  *                            the cookie will be deleted.
  *                            If set to null or omitted, the cookie will be a session cookie
- *                            and will not be retained when the the browser exits.
+ *                            and will not be retained when the browser exits.
  * @param {String} [options.path] Restrict access of a cookie to particular directory
  *                               (default: path of page that created the cookie).
  * @param {String} [options.domain] Override what web sites are allowed to access cookie