diff --git a/activemodel/lib/active_model/validations/confirmation.rb b/activemodel/lib/active_model/validations/confirmation.rb
index 2c8a840124cff9feab278d4b1a842c945cec58ff..00df10cef00da16ad90c000ca74105d089106daf 100644
--- a/activemodel/lib/active_model/validations/confirmation.rb
+++ b/activemodel/lib/active_model/validations/confirmation.rb
@@ -10,7 +10,9 @@ def validate_each(record, attribute, value)
       end
 
       def setup(klass)
-        klass.send(:attr_accessor, *attributes.map { |attribute| :"#{attribute}_confirmation" })
+        klass.send(:attr_accessor, *attributes.map do |attribute|
+          :"#{attribute}_confirmation" unless klass.method_defined?(:"#{attribute}_confirmation")
+        end.compact)
       end
     end