It is worth nothing that rsync doesn't compare just by size and mtime but also (relative) path - i.e. it normally compares an old copy of a file with the current version of the same file. So the likelyhood of "collisions" is much smaller than a file de-duplicating tool that compares random files.
I think you may misunderstand what httm does. httm prints the size, date and corresponding locations of available unique versions of files residing on snapshots.
And -- this makes it quite effective at proving how often this happens:
