How to delete redundant messages in Outlook
The Conversation Clean Up feature in Outlook can reduce the number of messages in your mail folders. Redundant messages throughout a Conversation are moved to the Deleted Items folder.
A Conversation is the complete set of email messages from the first message through all responses. The messages of a Conversation have the same subject.
Conversation Clean Up evaluates the contents of each message in the Conversation. If a message is completely contained within one of the replies, the previous message is deleted.