Reflinks with BTRFS and ZFS start off with behavior similar to hardlinks, but when you modify one of the files it doesn’t change the other one. Hard links share the same inode and edits to one file must affect the other.
Example:
File A: 10GB
File B: Reflink’d to File A
10GB total space used, 10GB shared between A and B.
Modify 1GB of File A
1GB of new blocks created, 11GB total now used. 9 GB still shared between A and B.