Yes. Hyperspace is finding the identical files and then replacing all but one copy with a reflink copy using the filesystem's reflink functionality.

When you asked about the filesystem, I assumed you were asking about which filesystem feature was being used, since hyperspace itself is not provided by the filesystem.

Someone else mentioned[0] fclones, which can do this task of finding and replacing duplicates with reflinks on more than just macOS, if you were looking for a userspace tool.

[0]: https://news.ycombinator.com/item?id=43173713

Hyperspace uses built in APFS features, it just applies them to existing files.

You only get CoW on APFS if you copy a file with certain APIs or tools.

If you have a program that does it manually, you copied a duplicate to somewhere on your desk from some other source, or your files already existed on the file system when you converted to APFS because you’ve been carrying them for a long time then you’d have duplicates.

APFS doesn’t look for duplicates at any point. It just keeps track of those that it knows are duplicates because of copy operations.

You can do the same with `cp -c` on macOS, or `cp --reflink=always` on Linux, if your filesystem supports it.