Doodly videos can be duplicated when in your main Doodly dashboard. Simply select the video you would like to duplicate, right click on the title and select Duplicate Video:
The duplicated video will then be added at the very end of your project list as a copy with all of the properties of the original project:
To duplicate a scene when you are working within that scene click on the "+" icon. Then, drag the end of the scene out to extend it as needed.