Less known Outlook (starting from version 2010) feature is built-in duplicate email removal. Using this feature redundant email messages are removed to Deleted Items.
To access this feature select Outlook mail folder, go to Home tab > Delete section > Delete > Clean Up.
Now you should have 3 choices:
Clean Up Conversation – Selected Conversation is deduped.
Clean Up Folder – All Conversations in the selected folder are deduped.
Clean Up Folder & Subfolders – All Conversations in the selected folder and any subfolders are deduped.
Unfortunately, this only works for emails and does not allow much customization.