diff --git a/activesupport/lib/active_support/core_ext/object/duplicable.rb b/activesupport/lib/active_support/core_ext/object/duplicable.rb

index d4bf8ce..58045d5 100644

--- a/activesupport/lib/active_support/core_ext/object/duplicable.rb

+++ b/activesupport/lib/active_support/core_ext/object/duplicable.rb

@@ -73,7 +73,8 @@ def duplicable?

 class Symbol

   begin

-    :symbol.dup

+    :symbol.dup # Ruby 2.4.x.

+    'symbol_from_string'.to_sym.dup # Some symbols can't `dup` in Ruby 2.4.0.

   rescue TypeError

     # Symbols are not duplicable: