Fritz said "I want to make a duplicate of a complete project."
My "answer" has worked fine for me, for many years, using Windows.
I am blessed with the initials "TKB". I say "blessed" because since 1968, those letters do not seem to have been used for other things.
Thus I can create a folder called KiCadTKB, and know that it is "MyKiCad"... and actually MY stuff, not a folder created by the system.
Within that, I create a "meta folder" for every project. (And every project has a short-form name, e.g. "PCB274". The Meta-folder name will usually have some additional hints as to what the project is for, e.g. "PCB274-quad optocoupler for level shifting" because PCB274 was a breakout to provide 4 channels of opto isolation to convert some 12v digital signals to Arduino-friendly levels.
Within that, a folder called PCB274. That will be my "latest, greatest" version of the board.
From time to time, I go into the ordinary comes-with-Windows file manager, "File Explorer". (I can do all of the following mid-session, without shutting down KiCad. I need to have saved (from within KiCad) any open files.) I right-click on the PCB274 file. Select "Copy". Right-click on a blank part of "PCB274-quad optocoupler for level shifting"' window. Select "Paste". That gives me my duplicate of my project, named "PCB274-Copy" for the moment. I would change the name to something like "PCB274BU19b22-1405", if the backup were made in 2019, November (a/b/c for Oct/Nov/Dec, so month is always a single character), 5th day, at 2:05pm.
If the backup stores the project in the state it was in when I posted it online, or set it to OSHPark.com who make my PCBs, then that would be indicated in the name after the datestamp.
It Just Works. (And the same approach works in many contexts. A few... Arduino... are fussy about the name of the lowest level "container" folder, but renaming it back to what it was is simple enough, if you need to revert to a backup. Or you can add an extra layer of folders, to have "Ardu 384BU19b22-1405" hold just a folder "Ardu384", with the project in THAT.)
