pyyaml duplicate keys patch

gistfile1.txt

diff --git a/lib/yaml/constructor.py b/lib/yaml/constructor.py
index 516dad1..598963c 100644
--- a/lib/yaml/constructor.py
+++ b/lib/yaml/constructor.py
@@ -136,6 +136,9 @@ class BaseConstructor(object):
             except TypeError, exc:
                 raise ConstructorError("while constructing a mapping", node.start_mark,
                         "found unacceptable key (%s)" % exc, key_node.start_mark)
+            if key in mapping:
+                raise ConstructorError("while constructing a mapping", node.start_mark,
+                        "found duplicate key (%s)" % key, key_node.start_mark)
             value = self.construct_object(value_node, deep=deep)
             mapping[key] = value
         return mapping
diff --git a/lib3/yaml/constructor.py b/lib3/yaml/constructor.py
index 34fc1ae..8ab6b36 100644
--- a/lib3/yaml/constructor.py
+++ b/lib3/yaml/constructor.py
@@ -132,6 +132,9 @@ class BaseConstructor:
             if not isinstance(key, collections.abc.Hashable):
                 raise ConstructorError("while constructing a mapping", node.start_mark,
                         "found unhashable key", key_node.start_mark)
+            if key in mapping:
+                raise ConstructorError("while constructing a mapping", node.start_mark,
+                        "found duplicate key (%s)" % key, key_node.start_mark)
             value = self.construct_object(value_node, deep=deep)
             mapping[key] = value
         return mapping