#!/usr/bin/env perl

use strict;
use warnings;

my %hdrs;
my $hdr;
my @hdrs_order;
my %hdrs_dup = (
    "Content-Disposition", 1,
    "Date" => 1,
    "From" => 1,
    "Message-ID" => 1,
    "Subject" => 1,
    "To" => 1,
);

while (<>) {
    chomp;
    last if ($_ eq "");
    /^([^:]+): (.*)$/;
    if (/^\s/) {
        @{$hdrs{$hdr}}[-1] .= "\n$_";
        next;
    };
    $hdr = $1;
    my $val = $2;
    (defined $hdrs{$hdr}) ? (push @{$hdrs{$hdr}}, $val) : ($hdrs{$hdr} = [$val]);
    push @hdrs_order, $hdr;
};

my %seen;
foreach (@hdrs_order) {
    next if (defined $seen{$_});
    $seen{$_} = 1;
    if (defined $hdrs_dup{$_}) {
        print "$_: @{$hdrs{$_}}[-1]\n";
        next;
    };
    foreach my $val (@{$hdrs{$_}}) {
        print "$_: $val\n";
    };
};

print "\n";
while (<>) { print };