[exim] Automatic sent folder update

Hello!

    I'm trying to configure exim to automatically update Imap Sent folders.
It almost works, but the exim writes one copy of outgoing email for each
recipient, when I'd like to have only one.
    If I understand right, I need to add some kind of "first_delivery"
condition, but I have no idea how. Can someone help me ?

# ROUTER

sentupdate:
driver = accept
domains = +local_domains
senders = ${lookup pgsql{...}{$value}}
transport = sent_backup
unseen = true

# TRANSPORT

sent_backup:
driver = appendfile
maildir_format = true
create_directory = true
directory = ${lookup pgsql{...}{$value}}


best regards
Jarek