Please note that you can merge any notebook into any other notebook, regardless of their relationship (or parent author). See this comment for the detailed steps:
You can already copy a notebook without creating a fork, because any two notebooks can be merged . The steps are not intuitive, so bear with me:
Open the notebook that you want to copy. This is the ORIGINAL. Access the notebook ID:
for private or shared notebooks: The part in the URL after “/d/”.
for published notebooks: Access the notebook history; the ID is the part of the URL between “/d/” and “@”.
Create a new notebook, save the first cell. This is the COPY.
Open the URL https://observab…