Fixes #3811 - Able to create custom fields for existing relation (e. g. ticket.state) - will lead to non bootable Zammad.

app/models/object_manager/attribute.rb

@@ -820,7 +820,7 @@ is certain attribute used by triggers, overviews or schedulers
     end
 
     record = object_lookup.name.constantize.new
-    if record.respond_to?(name.to_sym) && record.attributes.key?(name) && new_record?
+    if new_record? && (record.respond_to?(name.to_sym) || record.attributes.key?(name))
       errors.add(:name, "#{name} already exists!")
     end
 

spec/models/object_manager/attribute_spec.rb

@@ -76,6 +76,22 @@ RSpec.describe ObjectManager::Attribute, type: :model do
       end
     end
 
+    %w[title tags].each do |not_editable_attribute|
+      it "rejects '#{not_editable_attribute}' which is used" do
+        expect do
+          described_class.add attributes_for :object_manager_attribute_text, name: not_editable_attribute
+        end.to raise_error(ActiveRecord::RecordInvalid, 'Validation failed: Name Attribute not editable!')
+      end
+    end
+
+    %w[priority state note number].each do |existing_attribute|
+      it "rejects '#{existing_attribute}' which is used" do
+        expect do
+          described_class.add attributes_for :object_manager_attribute_text, name: existing_attribute
+        end.to raise_error(ActiveRecord::RecordInvalid, "Validation failed: Name #{existing_attribute} already exists!")
+      end
+    end
+
     it 'rejects duplicate attribute name of conflicting types' do
       attribute = attributes_for :object_manager_attribute_text
       described_class.add attribute