I have had this exact problem too. When I duplicate a webpage file it copies the old url with it rather than generating a new one. Older urls are also stopping working once I get the "This is your first time re-publishing your link in the new Adobe Express, your URL has changed" message. Is there a solution?
I have tried three times to duplicate a webpage. Each time when I publish the page I get the same url that was used on the page I duplicated from. This is a new defect since the recent upgrade of Express. Before, when I duplicated a webpage the duplicate would have a new unique url. HELP PLEASE!