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

def validate_group(images):
        """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
        """
        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)