exim backup outgoing mail

Question

I have the following transport for backup of outgoing mail:

sent_backup:
  driver = appendfile
  delivery_date_add
  envelope_to_add
  return_path_add
  group = mail
  user = mail
  mode = 0660
  maildir_format = true
  create_directory = true
  directory_mode = 700
  directory = /var/mail/${sender_address}/Maildir/.Sent

The problem is, that exim duplicates emails sent to several recipients. Is there any solution to avoid this ?