I hacked this together back in December. It's a patch on GrandPerspective that sets the file size to zero for multiply-referenced files in your Time Machine backup. So you end up with only the files that have been backed up once. This is really useful for looking for files that are unintentionally filling up your Time Machine backup because they're large, and they change by small amounts constantly.
You can download it here.
Note that I did very little of the work here, most of it was done by the author of Grand Perspective.