diff --git a/activemodel/lib/active_model/dirty.rb b/activemodel/lib/active_model/dirty.rb
index d32791382470ef855fd35f6f3059f7ab706089be..7f7fb90d87fcc4965f3c2c58570806394b3e59ae 100644
--- a/activemodel/lib/active_model/dirty.rb
+++ b/activemodel/lib/active_model/dirty.rb
@@ -151,13 +151,15 @@ def attribute_was(attr)
 
       # Handle <tt>*_will_change!</tt> for +method_missing+.
       def attribute_will_change!(attr)
+        return if attribute_changed?(attr)
+
         begin
           value = __send__(attr)
           value = value.duplicable? ? value.clone : value
         rescue TypeError, NoMethodError
         end
 
-        changed_attributes[attr] = value unless changed_attributes.include?(attr)
+        changed_attributes[attr] = value
       end
 
       # Handle <tt>reset_*!</tt> for +method_missing+.