Feature summary (what you would like to be able to do and where):
Have a tracking category (Special:TrackingCategories) for files with duplicates.
Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
On enwiki, find files to tag for CSD F8 (available on Wikimedia Commons). Also to find files that were mistakenly exported to Commons so they can be deleted from Commons.
Benefits (why should this be implemented?):
https://www.google.com/search?q="The+following+file+is+a+duplicate+of+this+file"+-"requests+that+a+local+copy+of+this+file+be+kept"+site%3Aen.wikipedia.org is just silly. Special:ListDuplicatedFiles doesn't consider duplicates on Commons. Categories are easier to work with than lists. (search etc)