this is the code to prevent the "duplication"

gistfile1.txt

add_action('after_setup_theme', 'remove_custom_thumbnails', 30);
function remove_custom_thumbnails(){
    remove_image_size('tc-rectangular-size'); //or uncheck the proper option
    remove_image_size('tc-thumb');
    remove_image_size('slider-full');
    remove_image_size('slider');
}