Have you tried to remove duplicate MBOX items? I have a good way to remove MBOX Duplicate files on "SameTools MBOX Duplicate Remover". This tool helps when you want to remove duplicate emails and attachments from all folders of MBOX. You just need to follow a few steps and eliminate all your duplicate MBOX files in a simple way, and there is no need to install any other application. This software can support all Windows OS versions like 11, 10, 8.1, 8, 7, and aslo supports.