# code to mimic uniq(1)

##</code><code>##

# mode: divine
my @result = do {
  my %counter;
  grep ++$counter{$_} == 2, @input;
};

##</code><code>##

# mode: tired
for (@input) {
  $multiples{$_}++;
}
for keys( %multiples ) { 
  print "$_\n" if (--$multiples{$_});
}