Remove doubled words from documentation and comments.

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

Thanks to Jim Meyering for the original patch.