The example below shows an email with the subject "Enron Update Regarding the Testing Schedule". Using the Advanced Search view, you can create your search for the subject line of this email and exclude the system tag - DUPE:
Step 3: The results window will show all files that satisfy this search. You can sort by date: Recent first
Step 4: (a) Check the top-left checkbox to select all items in the results listing, then (b) uncheck the first result from the results page, leaving the remaining files selected.
Step 5: Tag the remaining checked files to include them in a new "duplicates" tag: in the Actions dialog box on the right, click on + Tag. (You will need to create this tag. Click here for more information on creating tags from search results.)
Step 6: Using the "duplicates" tag you created, you can exclude this tag from searches or from review sets. Here's an example of the parameter to add (with the AND connector) to any advanced search query to omit these files
Here is some more detailed information on excluding tags from searches, and also about using review sets: