FIX: Remove duplicate enrolled students in convenor view

When using enrol modal in convenor view, enrolling a student would duplicate them in list of students

src/app/common/services/unit-service.coffee

@@ -96,7 +96,12 @@ angular.module("doubtfire.common.services.units", [])
     unit.addStudent = (student) ->
       analyticsService.event 'Unit Service', 'Added Student'
       unit.extendStudent(student)
-      unit.students.push(student)
+      if not unit.students[unit.students.indexOf(unit.findStudent(student.project_id))]?
+        #handles adding a newly enrolled student tutor view student list with enrol modal
+        unit.students.push(student)
+      else
+        #handles not adding a new entry to the student list in the convenor view with enrol modal
+        unit.students[unit.students.indexOf(unit.findStudent(student.project_id))] = student
 
     unit.active_students = () ->
       _.filter unit.students, (student) -> student.enrolled