To remove duplicates when two emails are identical except for attachments, you can try SameTools MBOX Duplicate Remover. This tool can remove multiple duplicate MBOX emails and attachments during the finds. Keep important information/data with this tool, and they don't need to install any application. This ensures duplicates MBOX are removed with a few simple steps, and they support all Windows OS platforms, including 11, 10, 8.1, 8, and 7. Try their free demo.

Visit here: http://www.sametools.com/duplicate/mbox/