뻔한 = obvious (by saying 뻔한 twice) 뻔하디 뻔한 = sooooooo obvious + 뻔하디 = could say adv form of 뻔한(which is adj form of 뻔하다(verb)) but could only be used in that idiom. so just memorize '뻔하디 뻔한' as 'sooooo obvious'