Outlook’s Conversation View groups messages and their replies under a single heading so that you can easily follow the string of emails that have the same Subject. When emails are replied to again and again, the same message can appear in each Reply.
To delete repeated messages in the Conversation, you can use the Clean Up Conversation tool on the Home tab in the Delete group. Outlook will remove all of the repetitive quoted parts of the messages.

To delete the entire existing Conversation, as well as all future message with the same Subject, you can use the Ignore tool. Outlook will move the existing thread and all future messages to the Deleted Items folder.

To make Outlook do more work for you, take a training class from AETC