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: