Currently (June 2024) there is no one-button solution to duplicate a novel. In order to duplicate your novel, we would recommend you.
Create a new novel. In the template section, you can import the codex and other details that you may wish to.
At the bottom of the plan interface, select import to import your novel.
You will still need to add the codex images and scene summaries into your work, however everything else will have copied over to your new project.
You can 'upvote' the request for codex imports and for improved export/importing on our feedback portal.