code: def _no_duplicates_constructor(loader, node, deep=False):
      
  
      mapping = {}
      for key_node, value_node in node.value:
          key = loader.construct_object(key_node, deep=deep)
          value = loader.construct_object(value_node, deep=deep)
          if key in mapping:
              raise ConstructorError("while constructing a mapping",
                                     node.start_mark,
                                     "found duplicate key (%s)" % key,
                                     key_node.start_mark)
          mapping[key] = value
  
      return loader.construct_mapping(node, deep)
docs: Check for duplicate keys.