Duplicated images in Mailbox Posting (v5.1.8)

Hello, have you tested mailbox posting with multiple images?
Somehow images other than post cover are duplicated. (Screenshot attached.)

Mailbox publishing options:
- Save attachment: Yes
- Process Blog image: Yes

P.S.
There had been similar issue on version prior to 5.x.
https://stackideas.com/forums/incorrectly-duplicated-pictures-in-mailbox-posting
But it seems to be slightly different problem this time.


Thanks,
rikao