#!/bin/sh
set -e
folder -create +dupl-demo                                                
list_id='list-id: list.example.com'
message_id='message-id: a@example.org'

for i in 1 2 3 4 5 6 7 8 9 10; do
  if test $(($i%3)) -eq 0; then
    the_list_id="$list_id"
  else
    the_list_id=
  fi
  printf '%s\n%s\n' "$message_id" "$the_list_id" > "$(mhpath +dupl-demo new)"
done
rmm +dupl-demo $(dupl)
scan +dupl-demo -format '%(msg) %{message-id} %{list-id}'