getting LDAP_ALREADY_EXISTS when trying to modify

[Giulia C. <gi...@be...>]

i'm using Net::LDAP::LDIF and Net::LDAP::Entry to attempt to modify 
directory entries, while bound to the directory as directory manager.

i get LDAP_ALREADY_EXISTS, which i don't understand--
isn't it true that an update using changetype: modify will overwrite the 
existing value for the attribute?
 
i'd really appreciate feedback on the following:

##################################
$debug = 1;

$ldap = Net::LDAP->new('ldapserver.mydomain.com') or die "$@";

$br = $ldap->bind ( #bind to directory with privs
                dn => 'cn=Special Someone',
                password => "blahblah"
                ) || die $@;

$bind_result = Net::LDAP::Util::ldap_error_name($br->code);
print "Result of attempt to bind was $bind_result \n" if $debug;

$ldif_final = "ldif.final";
$updateldif = Net::LDAP::LDIF->new("$ldif_final", "r") or die "$@";
        while( $finalentry = $updateldif->read() ) { 
        $final_dn = $finalentry->dn;
        @final_attrs = $finalentry->attributes;
        print "final dn of entry is $final_dn\n" if $debug;
        print "final attributes of entry are @final_attrs\n" if $debug;

        $ur = $finalentry->update( $ldap ); #update directory server
        $update_result = Net::LDAP::Util::ldap_error_name($ur->code);
        print "The result of the modify of $final_dn is: $update_result \n" if
$debug;

        }

$ldap->unbind();
##################################




% cat ldif.final
dn: uid=sally, ou=People, dc=mydomain, dc=com
changetype: modify
replace: managerchain
managerchain: joe lisa john bill larry


thanks,

giulia