only enroll a given course once

common/djangoapps/student/views.py

@@ -508,9 +508,10 @@ def course_info(request, course_id):
 @ensure_csrf_cookie
 def enroll(request, course_id):
     course = check_course(course_id, course_must_be_open=False)
-    
+
     user = request.user
-    enrollment = CourseEnrollment(user=user,
-        course_id=course.id)
-    enrollment.save()
+    if not CourseEnrollment.objects.filter(user=user, course_id=course.id).exists():
+        enrollment = CourseEnrollment(user=user,
+            course_id=course.id)
+        enrollment.save()
     return redirect(reverse('dashboard'))