how to merge two records according one field value about fluent-plugin-concat HOT 10 CLOSED

<source>
  @type dummy
  dummy [
   {"date": "2018-10-10", "id": "unique_id_01", "payload": "start"},
   {"date": "2018-10-10", "id": "unique_id_02", "payload": "start"},
   {"date": "2018-10-10", "id": "unique_id_01", "payload": "stop success"},
   {"date": "2018-10-10", "id": "unique_id_02", "payload": "stop fail"}
  ]
  tag dummy
  @label @INPUT
</source>

<label @INPUT>
  <filter dummy>
    @type concat
    key payload
    stream_identity_key id
    multiline_start_regexp /^start/
    flush_interval 5
    timeout_label @NORMAL
  </filter>
  <match dummy>
    @type relabel
    @label @NORMAL
  </match>
</label>

<label @NORMAL>
  <match dummy>
    @type stdout
  </match>
</label>

2018-10-16 18:35:21.030265578 +0900 dummy: {"date":"2018-10-10","id":"unique_id_01","payload":"start\nstop success"}
2018-10-16 18:35:22.031409080 +0900 dummy: {"date":"2018-10-10","id":"unique_id_02","payload":"start\nstop fail"}
2018-10-16 18:35:25.034645575 +0900 dummy: {"date":"2018-10-10","id":"unique_id_01","payload":"start\nstop success"}
2018-10-16 18:35:26.035902926 +0900 dummy: {"date":"2018-10-10","id":"unique_id_02","payload":"start\nstop fail"}

<source>
  @type tail
  path /path/to/log
  <parse>
    @type regexp
    expression /(?<date>2018-10-10) (?<unique_id>unique_id_\d\d) (?<message>.+)/
  </parse>
</source>

<filter>
  @type concat
  key message
  stream_identity_key unique_id
  multiline_start_regexp /^start/
</filter>

<source>
  @type tail
  # ... snip...
  tag audit
</source>
<filter audit>
  @type stdout # check parsed log
</filter>
<filter audit>
  @type concat
  # ... snip ...
</filter>
<filter audit>
  @type stdout # check concatenated log
</filter>
# ... snip ...
<match audit>
  @type copy
  <store>
    @type stdout
  </store>
  <store>
    @type elasticsearch
    # ... snip ...
  </store>
</match>