freeze the constant value so it isn't duped when used as a hash key

activerecord/lib/active_record/attribute_methods.rb

@@ -23,7 +23,7 @@ module AttributeMethods
       def self.set_name_cache(name, value)
         const_name = "ATTR_#{name}"
         unless const_defined? const_name
-          const_set const_name, value
+          const_set const_name, value.dup.freeze
         end
       end
     }