so, you cannot copy screens in AI but if you
- download the project as an AIA file (My projects, project, export selected project as AIA)
- then open the AIA file as a zip (7zip is a good program to use)
- open the src folder
- the appinventor folder
- the next folder with your login name
- then the name of your app
you will then see 3 files with the name you used for the screen (default is screen1). the files are a .yail, .scm and .bky. Just extract these files, rename them as something else (if file is screen1.yail, then rename to screen2.yail). Once all 3 files are renamed, just add them bad to the aia file through the zip program (7zip is good to use) and import the AIA file back into App Inventor online (you may need to rename the AIA file if you still have the original project saved on app inventor). You will basically have a new project with 2 identical pages.