Raise exception on duplicate keys

lib/psych/handlers/document_stream.rb

@@ -18,6 +18,18 @@ def end_document implicit_end = !streaming?
         @last.implicit_end = implicit_end
         @block.call pop
       end
+
+      def end_mapping
+        mapping = pop
+        keys = {}
+        mapping.children.each_slice(2) do |(key_scalar, _)|
+          next if key_scalar.is_a?(Psych::Nodes::Sequence) or key_scalar.is_a?(Psych::Nodes::Alias) or key_scalar.is_a?(Psych::Nodes::Mapping)
+          key = key_scalar.value
+          raise Psych::Exception, "Duplicate key #{key} exists on this level" if keys.key? key
+          keys[key] = nil
+        end
+        mapping
+      end
     end
   end
 end

test/psych/test_hash.rb

@@ -92,6 +92,16 @@ def test_map
       assert_equal X, x.class
     end
 
+    def test_error_on_same_key
+      assert_raises(Psych::Exception) do
+        Psych.load <<-EOF
+        -
+          same_key: 'value'
+          same_key: 'value'
+        EOF
+      end
+    end
+
     def test_self_referential
       @hash['self'] = @hash
       assert_cycle(@hash)