In the XML in the topic you mentioned (Find Image on Screen using Clipboard Image?) you can see there is a Unique field set to true.
Run the action twice, once with Unique set to true and once with Unique set to false, and use two different result variables (so use two slightly different XML with false and Location1 and true and Location2).
If Location1 is empty, then the image is nowhere on the screen.
If Location2 is not empty then there is a unique image on the screen.
If Location1 is not empty and Location2 is empty, then there is more than one copy on the screen.