You want to clean up your duplicate MBOX files before archiving without accidentally deleting important data. Then you can use SameTools MBOX Duplicate Remover to safely clean your duplicate MBOX files before you archive them. It scans your duplicate MBOX files like emails, and multiple attachments, and detects all duplicate MBOX items. It allows you to preview them before you remove duplicate MBOX files. Since it doesn't affect the original data, you can remove only unwanted duplicates from MBOX files while keeping the items you want to keep. Once you've cleaned your duplicate MBOX, it will be cleaned. You can now try their free demo.