My thought: use a hash. 
#!/usr/bin/perl -w
use strict;
my %message_id;

while ( <DATA> ) {
    if ( /Email address:/ ) {
        print;
    } elsif ( /Message-Id:/ && ! $message_id{ $_ }++ ) {
        print;
    }    
}

__DATA__
Email address:     19324213@glerp  
Message-Id: <200103160515.WAA07878@mal.elp.rr.com>
Message-Id: <200103160515.WAA07878@mal.elp.rr.com>
Email address:     19075680@glerp
Message-Id: <200103150512.VAA27508@prxy3.ba.best.com>
Message-Id: <200103150512.VAA27508@prxy3.ba.best.com>

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Cheers,
Ovid

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In reply to (Ovid - remove duplicates) Re: Seeing double by Ovid
in thread Seeing double by amearse

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