If you have duplicate MSG files, you can remove them by comparing the sender, recipient, subject and date of each email. I tried this manually with a small set, but it became difficult to track repeated messages when the number of files increased.
For a larger MSG collection, GainTools MSG Duplicate Remover can be used to scan and remove duplicate email items without Outlook. In my case, the remaining MSG files retained their message details and attachments.
Keep a backup of the original files before starting the cleanup. For a few MSG files, manual checking is fine, while bulk collections are easier to handle with a dedicated duplicate removal utility.