Merge pull request #198 from dementrock/master

fixed the bug that you can enroll a course several times

common/djangoapps/student/views.py

@@ -510,6 +510,7 @@ def enroll(request, course_id):
     course = check_course(course_id, course_must_be_open=False)
 
     user = request.user
+    if not CourseEnrollment.objects.filter(user=user, course_id=course.id).exists():
         enrollment = CourseEnrollment(user=user,
             course_id=course.id)
         enrollment.save()