How to avoid adding duplicate of element.Laravel, PHP

Shortly there are teachers and i have to attach them courses. Every course should be added once only(not twice). My code is working but only for element with id=1.And the other courses can be attached to a teacher more than once. I am writing the code in laravel. (php). I can't uderstand why it is working only for the first element(course($id=1)). Can you help me please. Where is my mistake?? Thank You in advance. Here is my code

public function addCourse($id, Request $request) {
        $teacher = Teacher::findOrFail($id);
        if($teacher->has('courses')->where('id', $request->get('course_id'))->get()->isEmpty()) {
            $teacher->courses()->attach($request->get('course_id'));

        }
         else {
            flash()->error('This Course has been added already!')->important();
        }


        return back();
    }

via Ostonov Sherzod