Saying that finder should sum file sizes in a way that accounts for this makes sense.

I'm not sure adding a separate "Sanity check" would help though. If the sanity check is that the reported file sizes shouldn't exceed the disk size, that's still a lie.

If you have 1000 duplicate 10GB files that are sharing storage blocks in the file system, that's either 10TB or 10GB of stored files depending on how you count it. Reporting 2TB because that's the size of the disk is a less accurate answer than either of those.