#!/usr/bin/perl

while (<DATA>){
  chomp;
  $h{$_}++;  
}

for ( sort grep { $h{$_} != 1 } keys %h ) {
  print "$_\n";
}

__DATA__
A(01) 
B(02) 
C(03) 
A(01) 
D(04) 
E(05)