That md5/hash trick is pretty cool. Here's a better version of the program (could be cleaner bit it works afaik). Using the Message-Id: would be faster but then we wouldn't need the md5. :)

#!/usr/bin/perl

# Simple program to remove duplicate email messages
# from an mbox file. This program only looks at the content
# of the message for uniqueness, not entire message with the headers.
# There is no file locking, use this program on a backup 
# of your mbox file.
# Enjoy.

use strict;
use warnings;
use Digest::MD5 qw(md5_hex);

#grab file names from the program parameters.
#and do some error checking.
my $from = shift @ARGV;
my $keep = shift @ARGV;
my $junk = shift @ARGV;

if ( $#ARGV != -1 || ! defined $junk ) {
    print STDERR "usage: $0 original clean junk\n";
    exit(-1);
}

my (%uniq, $msg);
my ($head, $body);
my $i = 0;
my $dups = 0;
my $nulls = 0;

$|++;

open (my $IN,  "<$from") || die "cannot open $from: $!";
open (my $KEEP, ">$keep")   || die "cannot open $keep $!";
open (my $JUNK, ">$junk")   || die "cannot open $junk $!";
while(<$IN>) {
    #emails in mbox files always begin with ^From 
    #when /^From / is matched, process the previous message
    #then start on this message
    if(m/^From /) {
    next if (!defined $msg || $msg eq "");
    #increment the counter for a status report
    $i++;
    #print a status report if necessary.
    #I like to do it this way
    print '.' if(($i % 50) == 0);
    if(($i % 1000) == 0) {
        print " $i, $dups duplicates, $nulls null messages found\n" 
    }
    #since evolution can give different headers on the same message,
    #only hash the body of the message, and use that to compare to oth
+er
    #emails. The entire message will be stored in the hash though.
    ($head, $body) = split /\n\n/, $msg, 2;
    #standard perl technique for removing duplicates, using hashes and
+ 
    #md5 files.
    if ( ! defined $body ) {
        $nulls++;
        print $JUNK $msg;
    } else {
        my $md5 = md5_hex($body);
        if ( !defined $uniq{$md5} ) {
        $uniq{$md5} = 1;
        print $KEEP $msg;
        } else {
        print $JUNK $msg;
        $dups++;
        }
    }
    
    #done processing the previous message, start the next message
    $msg = $_;
    } else {
    #current line didn't match /^From / so this line is part of the
    #middle of the current message. Just tack it on to the end.
    $msg .= $_;
    }
}
print "Done, $i messages, $dups duplicates, $nulls nulls\n";
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