fix code warning by adding guard before defining reader for instance variable

Author: mojavelinux

CHANGELOG.adoc

@@ -4,6 +4,13 @@
 This document provides a curated view of the changes to Asciidoctor Reducer in each release.
 For a detailed view of what has changed, refer to the {url-repo}/commits/main[commit history] on GitHub.
 
+== Unreleased
+
+=== Fixed
+
+* Don't define reader for source_header_attributes instance variable if already defined
+* Only catch RangeError when flattening list when using Opal runtime
+
 == 1.1.1 (2025-03-27) - @mojavelinux
 
 === Fixed

lib/asciidoctor/reducer/header_attribute_tracker.rb

@@ -3,6 +3,7 @@
 module Asciidoctor::Reducer
   module HeaderAttributeTracker
     def self.extended instance
+      return if instance.singleton_class.method_defined? :source_header_attributes
       instance.singleton_class.send :attr_reader, :source_header_attributes
     end