Every approach is this approach :) its like a search engine

You iterate over you files, and you index each file

To index, you pick a unit (ex one word, or three adjacent lines of code)

Generate a list of all units for a file

Normalize each unit. For words you would stem (remove prefix/suffix..) to find the root, for lines you would remove insignificant whitespace, insignificant commas... normalize quoting characters...

Hash each unit (sha1), and associate all this in a database

Then, to find duplication, query the database to find duplicate hashes

This is not unlike what git (git gc) does, so I wouldn't be surprised if git provides provided a tool to help you visualize these duplications, although I don't know of one

It goes without saying before making code changes, you need a comprehensive test suite :)