code: def validate_group(images):
          
          image_ids = set()
          for image in images:
              key = image.folder + image.name
              if key in image_ids:
                  raise ValueError('Duplicate images in group: ' + key)
              else:
                  image_ids.add(key)
docs: Validates that the combination of folder and name for all images in
          a group is unique. Raises a ValueError exception if uniqueness
          constraint is violated.
  
          Parameters
          ----------
          images : List(GroupImage)
              List of images in group