diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index c6535082d3a7b3bdc81430348b68ddebcbd396ce..5cd8f77f0d03781a1bfbd5938453d44140e5d6b8 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -78,6 +78,11 @@ def initialize(base)
       @messages = ActiveSupport::OrderedHash.new
     end
 
+    def initialize_dup(other)
+      @messages = other.messages.dup
+      super
+    end
+
     # Clear the messages
     def clear
       messages.clear
@@ -118,7 +123,7 @@ def [](attribute)
     #   p.errors[:name] = "must be set"
     #   p.errors[:name] # => ['must be set']
     def []=(attribute, error)
-      self[attribute.to_sym] << error
+      self[attribute] << error
     end
 
     # Iterates through each error key, value pair in the error messages hash.
diff --git a/activemodel/test/cases/errors_test.rb b/activemodel/test/cases/errors_test.rb
index 4edeece3e8c6bf986fa98eca4d3d54d3f446fc00..ab80f193b6b71e52b57b68df24e3df130691b549 100644
--- a/activemodel/test/cases/errors_test.rb
+++ b/activemodel/test/cases/errors_test.rb
@@ -40,6 +40,14 @@ def test_include?
     assert errors.include?(:foo), 'errors should include :foo'
   end
 
+  def test_dup
+    errors = ActiveModel::Errors.new(self)
+    errors[:foo] = 'bar'
+    errors_dup = errors.dup
+    errors_dup[:bar] = 'omg'
+    assert_not_same errors_dup.messages, errors.messages
+  end
+
   def test_has_key?
     errors = ActiveModel::Errors.new(self)
     errors[:foo] = 'omg'