File duplication takes no disk space!
Apple File System (APFS) and Clones

Copying a file creates a clone of that file. Clones take very little space. Only when a clone is modified, is significant additional space required.


  Clones
  
  Clones allow the operating system to make efficient file copies on the same volume without occupying additional storage space. Changes to a cloned file are saved as delta extents, reducing storage space required for document revisions and copies.[9] There is, however, no interface to mark two copies of the same file as clones of the other, or for other types of data deduplication.


See also iMore's Apple File System (APFS): What you need to know.

To understand how APFS works, see the Apple File System Reference.