omniauth: prevent duplicating accounts

Closes: #149

app/overrides/controllers/decidim/devise/omniauth_registrations_controller_override.rb

@@ -1,6 +1,23 @@
 Decidim::Devise::OmniauthRegistrationsController.class_eval do
   private
 
+  def user_params_from_oauth_hash
+    return nil if oauth_data.empty?
+
+    email = oauth_data.dig(:info, :email)
+    user = Decidim::User.find_by(email: email, organization: current_organization)
+
+    {
+      provider: oauth_data[:provider],
+      uid: oauth_data[:uid],
+      name: oauth_data[:info][:name] || user&.name,
+      nickname: oauth_data[:info][:nickname] || user&.nickname,
+      oauth_signature: Decidim::OmniauthRegistrationForm.create_signature(oauth_data[:provider], oauth_data[:uid]),
+      avatar_url: oauth_data[:info][:image],
+      raw_data: oauth_hash
+    }
+  end
+
   def verified_email
     @verified_email ||= oauth_data.dig(:info, :email) || params.dig(:user, :email)
   end