The easiest way is to do a batch run to produce a track database. You can use the Create Database Utility on the web resources page.

Once produced, you can open the database and do a Look for Duplicates from the context menu. You can specify criteria based on Title, Album, Artist, Track, Disc, Year and Length.

This will identify potential duplicates in the database. If you then wanted to save some text to the Comments field it could be done by an action run directly from the database.

Let me know if I've misunderstood what you want.