The ID card also has this amazing function where you can log in to sites using the card without revealing your identity, and even merging the databases from two sites does not allow two users to be identified as the same natural person: https://www.personalausweisportal.de/Webs/PA/EN/business/tec...
I have never seen a website offering login using this function, though ;-)
The other day I found out that not only ZFS and Btrfs have a dedup feature, but also XFS (which I use on my home partition) supports a feature called reflinks aka block sharing. So I threw duperemove at ~/.virtualenvs and ~/.cache/bazel and freed up a good 15GB of disk space. Good stuff.
I have never seen a website offering login using this function, though ;-)