Add a "duplicate notebook" option that keeps the old and new workbooks open

[JasonWeill]

The option to duplicate the current notebook doesn't exist any more. I'm guessing "Save as" is the replacement. However, after using this, whilst the newly named notebook pops up in a new tab correctly, the old tab no longer works -- the notebook disappears and the error "kernel unknown" appears. The desired behavior is to have both the new and old notebooks running in separate kernels in separate windows/tabs.

Originally reported by @jph00 in #7041 (comment).

In JupyterLab there is "Duplicate Notebook" option which was recently exposed in command palette and in the tab context menu jupyterlab/jupyterlab#12870. It appears to be an unintentional omission that it was left out in Notebook, it would be a good first issue to restore it by adding docmanager:duplicate to menus and command palette where needed (which might be as simple as just changing a few JSON files, see jupyterlab/jupyterlab#12870)

There is also a button in the tree view:

Reply posted by @krassowski in #7041 (comment).

[JasonWeill]

There is already a "duplicate" option in the context menu that appears when I right-click on a file in the tree view.