Outlook has introduced a feature to clean up conversations in a folder including, or excluding, its sub folders. This removes the repeated replies on an email thread, and keeps the latest mail intact. You can find the clean up under File- cleanup.
For further customization on the cleanup rules, go to Options - Mail - Conversation clean up section. You can set additional rules such as, "Don't move unread messages," and more.
Visit the DevX Tip Bank