Fix bug where users could get points for completing the same challenge.

controllers/courseware.js

@@ -258,7 +258,7 @@ exports.completedCourseware = function (req, res, next) {
   });
   var index = req.user.completedCoursewares.indexOf(coursewareHash);
 
-  if (index === -1) {
+  if (index > -1) {
     req.user.progressTimestamps.push(Date.now() || 0);
     req.user.uncompletedCoursewares.splice(index, 1);
   }