Patch/fix for review:

diff --git a/app/models/user.rb b/app/models/user.rb
index bacb31f..c75724b 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -325,6 +325,9 @@ class User < ApplicationRecord
         new_usergroups += auth_source.external_usergroups.includes(:usergroup).where(:name => external_groups).map(&:usergroup)
         user.update(Hash[attrs.select { |k, v| v.present? }])
         user.usergroups = new_usergroups.uniq
+      else
+        logger.info "Failed to create external User: Username '#{attrs[:login]}' already exists" 
+        user = nil
       end

       user
diff --git a/test/models/user_test.rb b/test/models/user_test.rb
index a72c492..f8adcff 100644
--- a/test/models/user_test.rb
+++ b/test/models/user_test.rb
@@ -718,11 +718,11 @@ class UserTest < ActiveSupport::TestCase
     not_existing_auth_source = 'new_external_source'

     context "internal or not existing AuthSource" do
-      test 'existing user' do
+      test 'existing user without auth source specified' do
         assert_difference('User.count', 0) do
           login = users(:one).login
-          assert_equal User.find_or_create_external_user({:login => login}, nil),
-            User.find_by_login(login)
+          user = User.find_or_create_external_user({:login => login}, nil)
+          assert user.nil?
         end
       end

@@ -762,6 +762,15 @@ class UserTest < ActiveSupport::TestCase
         end
       end

+      test 'existing user of different authsource' do
+        existing_source = AuthSourceExternal.where(:name => 'existing_source').first_or_create
+        existing_user = FactoryBot.create(:user, :auth_source => existing_source)
+        assert_difference('User.count', 0) do
+          user = User.find_or_create_external_user({:login => existing_user}, @apache_source.name)
+          assert user.nil?
+        end
+      end
+
       test "not existing with attributes" do
         created_user = User.find_or_create_external_user(
           {:login => not_existing_user_login,