diff --git a/app/controllers/admin/users_controller.rb b/app/controllers/admin/users_controller.rb
index c096cf078f6..ee03ac7058d 100644
--- a/app/controllers/admin/users_controller.rb
+++ b/app/controllers/admin/users_controller.rb
@@ -121,7 +121,10 @@ class Admin::UsersController < Admin::AdminController
   def add_group
     group = Group.find(params[:group_id].to_i)
     return render_json_error group unless group && !group.automatic
-    group.users << @user
+
+    # We don't care about duplicate group assignment
+    group.users << @user rescue ActiveRecord::RecordNotUnique
+
     render nothing: true
   end
 
diff --git a/spec/controllers/admin/users_controller_spec.rb b/spec/controllers/admin/users_controller_spec.rb
index 4f93fe55a55..20d5d198df0 100644
--- a/spec/controllers/admin/users_controller_spec.rb
+++ b/spec/controllers/admin/users_controller_spec.rb
@@ -37,7 +37,6 @@ describe Admin::UsersController do
           expect(UserHistory.where(action: UserHistory.actions[:check_email], acting_user_id: @user.id).count).to eq(0)
 
           xhr :get, :index, show_emails: "true"
-          data = ::JSON.parse(response.body)
 
           expect(UserHistory.where(action: UserHistory.actions[:check_email], acting_user_id: @user.id).count).to eq(1)
         end
@@ -173,6 +172,22 @@ describe Admin::UsersController do
       end
     end
 
+    context '.add_group' do
+      let(:user) { Fabricate(:user) }
+      let(:group) { Fabricate(:group) }
+
+      it 'adds the user to the group' do
+        xhr :post, :add_group, group_id: group.id, user_id: user.id
+        expect(response).to be_success
+
+        expect(GroupUser.where(user_id: user.id, group_id: group.id).exists?).to eq(true)
+
+        # Doing it again doesn't raise an error
+        xhr :post, :add_group, group_id: group.id, user_id: user.id
+        expect(response).to be_success
+      end
+    end
+
     context '.primary_group' do
       before do
         @another_user = Fabricate(:coding_horror)