macOS - Prevent same background image from showing on multiple Desktops?
The technique mentioned below would work if you do not set the Desktop background from the standard images provided in System Preferences.


Create a directory to hold the desired background images at some convenient location in your Home directory.
Within the created directory, create one directory for each open Space.
Place desired background images in each of the directories for Space. You can ensure that there are no duplicate images between directories.
Go to each Space one-by-one and choose custom directory for background image.
Set the background image for each Space from the respective directory.


Using this approach you can ensure there’s no duplication.