It is not possible to duplicate or merge existing projects in Rank Tracker at the moment.  

Projects can't be duplicated because it is not possible to have the same site with the exact root domain and mode added to your Rank Tracker dashboard.  

For example: The Rank Tracker won't allow app.ahrefs.com to be added a second time as this would mean the new project will share the same root domain as the existing project.  Instead, what will happen is the keywords and location added to the new project will merge into the existing one.