One somewhat dirty method might be to give images within each collection the same label color. With five colors available (actually six since no label is also a color) you can cover your five collections. After doing so for each collection, you can check the collections for 'false' colors, indicating the images appearing in more then one collection.
I am not sure I like this appraoch, since you would have to repeat the check after change in a collection. Also of course it is not applicable if you still need the labels for some other purpose.