$ ./input.pl 
HA
D2
D4
HC
D1
D3
$ cat ./input.pl 
#!/usr/bin/perl
# Script to print headers and duplicate items from input.txt

use warnings;
use strict;
my @records;
undef $/;

open ( INPUT, "< input.txt" ) || die "Couldn't open input file: $!\n";
# use a look-ahead assertion here
@records = split( /^(?=(?:H))/m, <INPUT> );
foreach my $record ( @records ) {
   my @lines = split( /\n/, $record );
   my $header = $lines[0];
   my %linehash;
   my $headerdone = 0;
   foreach my $line ( @lines ) {
      $linehash{$line}++;  
   }
   foreach my $key ( sort ( keys ( %linehash ) ) ) {
      my $value = $linehash{$key};
      if ( $value > 1 ) { 
         if ( $headerdone == 0 ) {
            printf( "%s\n", $header );
            $headerdone++;
         }
         printf( "%s\n", $key );
      }
   }
}
close ( INPUT );

exit ( 0 );

#! /opt/third-party/bin/perl

my ($content, $i, $header, $headerprint, %fileHash);
open(FILE, "< a") || die "Unable to open file : $!\n";

while( chomp($content = <FILE>) ) {
  if( $content =~ m/^H/ ) {
    $headerprint = 0;
    $header = $content;
    %fileHash = ();
  }
  else {
    if( $headerprint == 0 ) {
      print "$header\n"; $headerprint = 1;
    }
    print "$content\n" if exists $fileHash{$content};
    $fileHash{$content} = $i++;
  }
}

exit 0