diff --git a/app/controllers/users/registrations_controller.rb b/app/controllers/users/registrations_controller.rb
index f9abe0b35..b1787becf 100644
--- a/app/controllers/users/registrations_controller.rb
+++ b/app/controllers/users/registrations_controller.rb
@@ -15,7 +15,14 @@ class Users::RegistrationsController < Devise::RegistrationsController
 
   # POST /resource
   def create
-    super
+    user = User.find_by(email: params[:user][:email])
+    if user.present?
+      UserMailer.new_account_warning(user).deliver
+      flash.notice = t('devise.registrations.signed_up_but_unconfirmed')
+      redirect_to root_path
+    else
+      super
+    end
   end
 
   # GET /resource/edit
diff --git a/app/mailers/user_mailer.rb b/app/mailers/user_mailer.rb
new file mode 100644
index 000000000..2bef4f3ba
--- /dev/null
+++ b/app/mailers/user_mailer.rb
@@ -0,0 +1,8 @@
+class UserMailer < ApplicationMailer
+  layout 'mailers/layout'
+
+  def new_account_warning(user)
+    @user = user
+    mail(to: user.email, subject: "Création de compte")
+  end
+end
diff --git a/app/views/user_mailer/new_account_warning.html.haml b/app/views/user_mailer/new_account_warning.html.haml
new file mode 100644
index 000000000..3127c3674
--- /dev/null
+++ b/app/views/user_mailer/new_account_warning.html.haml
@@ -0,0 +1,14 @@
+- content_for(:title, 'Demande de création de compte')
+
+%h1 Bonjour
+
+%p Une demande de création de compte a été réalisée sur le site demarches-simplifiees.fr pour l'email #{@user.email}.
+%p
+  Votre compte existe déjà. Si vous souhaitez changer votre mot de passe, veuillez suivre les instructions à l'adresse suivante
+  #{link_to(new_password_url(@user), new_password_url(@user))}.
+%p Si vous n'êtes pas à l'origine de cette demande, vous pouvez ignorer ce mail.
+
+%p
+  Cordialement,
+  %br
+  L'équipe demarches-simplifiees.fr
diff --git a/spec/controllers/users/registrations_controller_spec.rb b/spec/controllers/users/registrations_controller_spec.rb
index 14f391b35..36d26d9de 100644
--- a/spec/controllers/users/registrations_controller_spec.rb
+++ b/spec/controllers/users/registrations_controller_spec.rb
@@ -1,5 +1,3 @@
-require 'spec_helper'
-
 describe Users::RegistrationsController, type: :controller do
   let(:email) { 'test@octo.com' }
   let(:password) { 'password' }
@@ -33,5 +31,18 @@ describe Users::RegistrationsController, type: :controller do
         subject
       end
     end
+
+    context 'when the user already exists' do
+      let!(:existing_user) { create(:user, email: email, password: password) }
+
+      before do
+        allow(UserMailer).to receive(:new_account_warning).and_return(double(deliver: 'deliver'))
+        subject
+      end
+
+      it { expect(response).to redirect_to(root_path) }
+      it { expect(flash.notice).to eq(I18n.t('devise.registrations.signed_up_but_unconfirmed')) }
+      it { expect(UserMailer).to have_received(:new_account_warning) }
+    end
   end
 end
diff --git a/spec/mailers/previews/user_mailer_preview.rb b/spec/mailers/previews/user_mailer_preview.rb
new file mode 100644
index 000000000..001799662
--- /dev/null
+++ b/spec/mailers/previews/user_mailer_preview.rb
@@ -0,0 +1,5 @@
+class UserPreview < ActionMailer::Preview
+  def new_account_warning
+    UserMailer.new_account_warning(User.first)
+  end
+end