Is it possible to have multiple screens in a scene? I've tried making a duplicate material, but there doesn't seem to be a way to duplicate the image node in order to have a second input image?
Yes! Your method results in a duplicate material that still uses the original node sub-groups. You could go through every group and make them unique, but it's not really feasible, so:
The quickest way I can think of is just appending the display and material from the MRMO-CRT .blend project into a new project, as many times as you need; this would create "proper" copies of the material (with its name ending in .001, .002 etc) with the subgroups properly copied.