diff --git a/common/djangoapps/course_groups/cohorts.py b/common/djangoapps/course_groups/cohorts.py
index 6c18491775..dfede1147a 100644
--- a/common/djangoapps/course_groups/cohorts.py
+++ b/common/djangoapps/course_groups/cohorts.py
@@ -117,6 +117,12 @@ def add_cohort(course_id, name):
                                           group_type=CourseUserGroup.COHORT,
                                           name=name)
 
+class CohortConflict(Exception):
+    """
+    Raised when user to be added is already in another cohort in same course.
+    """
+    pass
+
 def add_user_to_cohort(cohort, username_or_email):
     """
     Look up the given user, and if successful, add them to the specified cohort.
@@ -131,15 +137,29 @@ def add_user_to_cohort(cohort, username_or_email):
     Raises:
         User.DoesNotExist if can't find user.
 
-        ValueError if user already present.
+        ValueError if user already present in this cohort.
+
+        CohortConflict if user already in another cohort.
     """
     if '@' in username_or_email:
         user = User.objects.get(email=username_or_email)
     else:
         user = User.objects.get(username=username_or_email)
 
-    if cohort.users.filter(id=user.id).exists():
-        raise ValueError("User {0} already present".format(user.username))
+    # If user in any cohorts in this course already, complain
+    course_cohorts = CourseUserGroup.objects.filter(
+        course_id=cohort.course_id,
+        users__id=user.id,
+        group_type=CourseUserGroup.COHORT)
+    if course_cohorts.exists():
+        if course_cohorts[0] == cohort:
+            raise ValueError("User {0} already present in cohort {1}".format(
+                user.username,
+                cohort.name))
+        else:
+            raise CohortConflict("User {0} is in another cohort {1} in course"
+                                 .format(user.username,
+                                         course_cohorts[0].name))
 
     cohort.users.add(user)
     return user
diff --git a/common/djangoapps/course_groups/views.py b/common/djangoapps/course_groups/views.py
index e82532ae50..d64622dcb0 100644
--- a/common/djangoapps/course_groups/views.py
+++ b/common/djangoapps/course_groups/views.py
@@ -133,6 +133,8 @@ def add_users_to_cohort(request, course_id, cohort_id):
      'added': [{'username': username,
                 'name': name,
                 'email': email}, ...],
+     'conflict': [{'username_or_email': ...,
+                    'msg': ...}],  # in another cohort
      'present': [str1, str2, ...],    # already there
      'unknown': [str1, str2, ...]}
     """
@@ -146,6 +148,7 @@ def add_users_to_cohort(request, course_id, cohort_id):
     users = request.POST.get('users', '')
     added = []
     present = []
+    conflict = []
     unknown = []
     for username_or_email in split_by_comma_and_whitespace(users):
         try:
@@ -158,10 +161,15 @@ def add_users_to_cohort(request, course_id, cohort_id):
             present.append(username_or_email)
         except User.DoesNotExist:
             unknown.append(username_or_email)
+        except cohorts.CohortConflict as err:
+            conflict.append({'username_or_email': username_or_email,
+                              'msg': str(err)})
+
 
     return JsonHttpReponse({'success': True,
                             'added': added,
                             'present': present,
+                            'conflict': conflict,
                             'unknown': unknown})
 
 @ensure_csrf_cookie
diff --git a/common/static/js/course_groups/cohorts.js b/common/static/js/course_groups/cohorts.js
index ada0b16bd5..aa3ce34b5b 100644
--- a/common/static/js/course_groups/cohorts.js
+++ b/common/static/js/course_groups/cohorts.js
@@ -166,8 +166,10 @@ var CohortManager = (function ($) {
             function adder(note, color) {
                 return function(item) {
                     var li = $('<li></li>')
-                    if (typeof item === "object") {
+                    if (typeof item === "object" && item.username) {
                         li.text(note + ' ' + item.name + ', ' + item.username + ', ' + item.email);
+                    } else if (typeof item === "object" && item.msg) {
+                        li.text(note + ' ' + item.username_or_email + ', ' + item.msg);
                     } else {
                         // string
                         li.text(note + ' ' + item);
@@ -176,10 +178,13 @@ var CohortManager = (function ($) {
                     op_results.append(li);
                 }
             }
+            op_results.empty();
             if (response && response.success) {
                 response.added.forEach(adder("Added", "green"));
                 response.present.forEach(adder("Already present:", "black"));
+                response.conflict.forEach(adder("In another cohort:", "purple"));
                 response.unknown.forEach(adder("Unknown user:", "red"));
+                users_area.val('')
             } else {
                 log_error(response.msg || "There was an error adding users");
             }