diff --git a/app/assets/javascripts/discourse/controllers/not-activated.js.es6 b/app/assets/javascripts/discourse/controllers/not-activated.js.es6
index 1c38cc58e59..86afe2ddd7c 100644
--- a/app/assets/javascripts/discourse/controllers/not-activated.js.es6
+++ b/app/assets/javascripts/discourse/controllers/not-activated.js.es6
@@ -1,4 +1,5 @@
 import { ajax } from 'discourse/lib/ajax';
+import { popupAjaxError } from 'discourse/lib/ajax-error';
 import ModalFunctionality from 'discourse/mixins/modal-functionality';
 
 export default Ember.Controller.extend(ModalFunctionality, {
@@ -9,9 +10,13 @@ export default Ember.Controller.extend(ModalFunctionality, {
   },
 
   actions: {
-    sendActivationEmail: function() {
-      ajax('/users/action/send_activation_email', {data: {username: this.get('username')}, type: 'POST'});
-      this.set('emailSent', true);
+    sendActivationEmail() {
+      ajax('/users/action/send_activation_email', {
+        data: { username: this.get('username') },
+        type: 'POST'
+      }).then(() => {
+        this.set('emailSent', true);
+      }).catch(popupAjaxError);
     }
   }
 
diff --git a/app/controllers/session_controller.rb b/app/controllers/session_controller.rb
index 0a18b3c539a..8d4b2250bf5 100644
--- a/app/controllers/session_controller.rb
+++ b/app/controllers/session_controller.rb
@@ -6,6 +6,8 @@ class SessionController < ApplicationController
   skip_before_filter :redirect_to_login_if_required
   skip_before_filter :preload_json, :check_xhr, only: ['sso', 'sso_login', 'become', 'sso_provider', 'destroy']
 
+  ACTIVATE_USER_KEY = "activate_user"
+
   def csrf
     render json: {csrf: form_authenticity_token }
   end
@@ -276,6 +278,7 @@ class SessionController < ApplicationController
   end
 
   def not_activated(user)
+    session[ACTIVATE_USER_KEY] = user.username
     render json: {
       error: I18n.t("login.not_activated"),
       reason: 'not_activated',
@@ -303,6 +306,7 @@ class SessionController < ApplicationController
   end
 
   def login(user)
+    session.delete(ACTIVATE_USER_KEY)
     log_on_user(user)
 
     if payload = session.delete(:sso_payload)
diff --git a/app/controllers/users_controller.rb b/app/controllers/users_controller.rb
index b06e6721efa..0e429e8c8c1 100644
--- a/app/controllers/users_controller.rb
+++ b/app/controllers/users_controller.rb
@@ -567,13 +567,25 @@ class UsersController < ApplicationController
       RateLimiter.new(nil, "activate-min-#{request.remote_ip}", 6, 1.minute).performed!
     end
 
+    if (current_user && !current_user.staff?) ||
+       (params[:username] != session[SessionController::ACTIVATE_USER_KEY])
+
+      raise Discourse::InvalidAccess
+    end
+
     @user = User.find_by_username_or_email(params[:username].to_s)
 
     raise Discourse::NotFound unless @user
 
-    @email_token = @user.email_tokens.unconfirmed.active.first
-    enqueue_activation_email if @user
-    render nothing: true
+    session.delete(SessionController::ACTIVATE_USER_KEY)
+
+    if @user.active
+      render_json_error(I18n.t('activation.activated'), status: 409)
+    elsif @user
+      @email_token = @user.email_tokens.unconfirmed.active.first
+      enqueue_activation_email
+      render nothing: true
+    end
   end
 
   def enqueue_activation_email
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index aa6423dccec..1619309dd96 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -614,6 +614,7 @@ en:
     welcome_to: "Welcome to %{site_name}!"
     approval_required: "A moderator must manually approve your new account before you can access this forum. You'll get an email when your account is approved!"
     missing_session: "We cannot detect if your account was created, please ensure you have cookies enabled."
+    activated: "Sorry, this account has already been activated."
   post_action_types:
     off_topic:
       title: 'Off-Topic'
diff --git a/spec/controllers/users_controller_spec.rb b/spec/controllers/users_controller_spec.rb
index 746d26a0712..4bb4c871bc7 100644
--- a/spec/controllers/users_controller_spec.rb
+++ b/spec/controllers/users_controller_spec.rb
@@ -1403,10 +1403,29 @@ describe UsersController do
     context 'for an existing user' do
       let(:user) { Fabricate(:user, active: false) }
 
+      context 'for an activated account' do
+        it 'fails' do
+          active_user = Fabricate(:user, active: true)
+          session[SessionController::ACTIVATE_USER_KEY] = active_user.username
+          xhr :post, :send_activation_email, username: active_user.username
+
+          expect(response.status).to eq(409)
+
+          expect(JSON.parse(response.body)['errors']).to include(I18n.t(
+            'activation.activated'
+          ))
+
+          expect(session[SessionController::ACTIVATE_USER_KEY]).to eq(nil)
+        end
+      end
+
       context 'with a valid email_token' do
         it 'should send the activation email' do
+          session["activate_user"] = user.username
           Jobs.expects(:enqueue).with(:critical_user_email, has_entries(type: :signup))
           xhr :post, :send_activation_email, username: user.username
+
+          expect(session[SessionController::ACTIVATE_USER_KEY]).to eq(nil)
         end
       end
 
@@ -1418,13 +1437,17 @@ describe UsersController do
 
         it 'should generate a new token' do
           expect {
+            session["activate_user"] = user.username
             xhr :post, :send_activation_email, username: user.username
           }.to change{ user.email_tokens(true).count }.by(1)
         end
 
         it 'should send an email' do
+          session["activate_user"] = user.username
           Jobs.expects(:enqueue).with(:critical_user_email, has_entries(type: :signup))
           xhr :post, :send_activation_email, username: user.username
+
+          expect(session[SessionController::ACTIVATE_USER_KEY]).to eq(nil)
         end
       end
     end