teacher students professor students
school class teacher desk table
chalk borad class school teacher

##</code><code>##

use strict;
use warnings;

my %hash;

while(<DATA>){
chomp;
  $hash{$_}++ for split;  
}

print join $/ => sort keys %hash; 
print $/,$/, join $/ => grep {$_ if $hash{$_} > 1} sort keys %hash;
__DATA__
teacher students professor students
school class teacher desk table
chalk borad class school teacher

##</code><code>##

borad
chalk
class
desk
professor
school
students
table
teacher

##</code><code>##

class
school
students
teacher