[exim] Duplicate emails

We are getting duplicate emails. It seems to be the same senders,
however not every one of their emails duplicate. 5% of mails from
Charter.net will duplicate, 10% of mails from Cox will duplicate, 100%
of mails from Interfax.net will duplicate.

Interfax (a mail to fax provider) is the only sender that consistently
send duplicate emails. When I asked them to fix it, they said:

"it seems that we are having problems sending you the e-mails and then
getting confirmations from the mail server that it received them - and
that is causing us to resend the e-mails over and over."

Anyone have any ideas?

My exim.conf follows:
# /etc/exim/exim.conf
host_lookup = 0.0.0.0/0
spamd_address = 127.0.0.1 783
qualify_domain = schmerold.com
qualify_recipient = schmerold.com
smtp_receive_timeout = 10m
message_size_limit = 25M
smtp_accept_max = 50
smtp_banner = mx0.schmerold.com secure smtp server
domainlist relay_to_domains = partial-lsearch;/etc/exim/transport
hostlist relay_from_hosts = 127.0.0.1
domainlist local_domains =
acl_smtp_rcpt = acl_check_rcpt
acl_smtp_mime = acl_check_mime
acl_smtp_data = acl_check_content
never_users = root
av_scanner = clamd:127.0.0.1 3310
perl_startup = do '/etc/exim/exim_surbl.pl'

auto_thaw = 1d
ignore_bounce_errors_after = 1d
timeout_frozen_after = 3d

helo_allow_chars = _

############# ACCEPT SETTINGS#######################
begin acl

acl_check_rcpt:
accept hosts = :
deny local_parts = ^.*[@%!/|] : ^\\.

   accept local_parts = postmaster
          domains = +local_domains

# See
http://www-csit.phy.cam.ac.uk/cs-it/security/sender_reject_bydomain for
examples
deny message = Message rejected because $sender_host_address is
blocked locally.
log_message = Message rejected from $sender_host_address by
sender_reject_bydomain
senders = @@lsearch*;/etc/exim/blacklist

   accept domains = +relay_to_domains
           endpass
          message = relay not permitted at this server
          verify = recipient

accept hosts = +relay_from_hosts

   deny senders = :
           condition     = ${if ! eq{$recipients_count}{1}{1}}
           message       = Bounces must have only a single recipient
           log_message   = Another denied due to backscatter-Single 
Recipient

deny message = relay not permitted at this server

   deny message = Access denied - $sender_host_address listed by 
$dnslist_domain\n$dnslist_text
        dnslists = sbl-xbl.spamhaus.org : dnsbl.sorbs.net : 
bl.spamcop.net : \
        multi.surbl.org : spam.dnsbl.sorbs.net : bl.spamcannibal.org : \
        dnsbl-2.uceprotect.net : b.barracudacentral.org : psbl.surriel.com

acl_check_mime:

warn decode = default

   deny message = Blacklisted file extension detected
        condition = ${if match \
                         {${lc:$mime_filename}} \
                         {\N(\.exe|\.pif|\.bat|\.scr|\.lnk|\.com)$\N} \
                      {1}{0}}

    deny condition = ${if <{$message_size}{100k}{yes}{no}}
         set acl_m0 = ${perl{surblspamcheck}}
         condition = ${if eq{$acl_m0}{false}{no}{yes}}
         message = $acl_m0

     accept

acl_check_content:
    deny condition = ${if <{$message_size}{100k}{yes}{no}}
         condition = ${if eq{$acl_m0}{}{yes}{no}}
         set acl_m1 = ${perl{surblspamcheck}}
          condition = ${if eq{$acl_m0}{}{yes}{no}}
         set acl_m1 = ${perl{surblspamcheck}}
         condition = ${if eq{$acl_m1}{false}{no}{yes}}
         message = $acl_m1

     deny message = This message contains a virus or other harmful 
content ($malware_name)
                   condition = ${if <{$message_size}{175k}{1}{0}}
                   demime = *
                   malware = *

    deny condition = ${if <{$message_size}{100k}{yes}{no}}
         condition = ${if eq{$acl_m0}{}{yes}{no}}
         set acl_m1 = ${perl{surblspamcheck}}
         condition = ${if eq{$acl_m1}{false}{no}{yes}}
         message = $acl_m1

     warn message = X-Spam-Score: $spam_score
          spam = exim:true

     warn message = X-Spam-Report: $spam_report
          spam = exim:true

     warn message = Subject: **** SPAM **** $h_Subject
          spam = exim

     deny message = This message scored $spam_score spam points.
          condition = ${if <{$message_size}{975k}{1}{0}}
          spam = exim:true
          condition = ${if >{$spam_score_int}{70}{1}{0}}

     accept

############# ROUTER SETTINGS#######################
begin routers
redirect:
driver = redirect
data = ${lookup{$local_part@$domain}lsearch{/etc/exim/user_alias}}

internal:
   driver = manualroute
   domains = +relay_to_domains
     driver = manualroute
   domains = +relay_to_domains
   transport = remote_smtp
   route_data = ${lookup{$domain}partial-lsearch{/etc/exim/transport}}

external:
driver = dnslookup
domains = ! +relay_to_domains
transport = remote_smtp
ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
no_more

############# TRANSPORT SETTINGS####################
begin transports

remote_smtp:
driver = smtp

begin retry
*                      *           F,2h,15m; G,16h,1h,1.5; F,3d,8h