I agree, it's a little annoying. It would be nice if this was supported and/or layout inheritance was bought over from Construct Classic.
One way around this that I tend to use (which does require a little bit of fussing about) is:
1) Backup your project.
2) Browse to the project on your computer. There will be a folder called "Layouts".
3) Find the layout you want to duplicate. There will be a .xml and a .uistate.xml file, for example Level1.xml and Level1.uistate.xml
4) Copy and paste these two files, then rename them to the new layout. For example, rename the copies Level2.xml and Level2.uistate.xml.
5) Open Level2.xml and change the following line to the new level name:
6) One last step - Browse to the .caproj file in the root directory and open it with notepad. Scroll to the tag:
7) In here create a new line for your new level:
8) Restart C2 and it should display the new layout fully duplicated.
It might seem like a lot of steps, but it only takes a few seconds once you have done it a few times.
Hope this helps!