While I can understand your logic, we think that such a feature that would reflect different navigation based on the state of a page could produce a lot of confusion among majority of our users. You can see all pages, regardless of their state in the administration area (Pages section) on the left side. You can filter the pages also based on their current status (same section). As for not wanting to publish a site until it's complete, you can build your site on a staging server first and deploy it to a production server. Another way is to create initial website on your local machine and then publish it to the production server. Finally, you can implement your own navigational controls and specify the criteria there as to which pages should be visible in the navigation at which time.
I was not able to reproduce the problem with duplicating pages that are not published. Could you please provide us with some details on this issue.
1. Which version of Sitefinity are you using?
2. What are the permissions of the user trying to duplicate a page?
3. Is there a message you receive when you try to duplicate a page? Can you post us a screenshot?
I am sorry that Sitefinity cannot accommodate your needs out of box, but we'll try to help you as much as possible to find workarounds.
the Telerik team