def check_mapping_key(self, node, key_node, mapping, key, value):
        # type: (Any, Any, Any, Any, Any) -> bool
        """return True if key is unique"""
        if key in mapping:
            if not self.allow_duplicate_keys:
                mk = mapping.get(key)
                if PY2:
                    if isinstance(key, unicode):
                        key = key.encode('utf-8')
                    if isinstance(value, unicode):
                        value = value.encode('utf-8')
                    if isinstance(mk, unicode):
                        mk = mk.encode('utf-8')
                args = [
                    'while constructing a mapping',
                    node.start_mark,
                    'found duplicate key "{}" with value "{}" '
                    '(original value: "{}")'.format(key, value, mk),
                    key_node.start_mark,
                    """
                    To suppress this check see:
                        http://yaml.readthedocs.io/en/latest/api.html#duplicate-keys
                    """,
                    """\
                    Duplicate keys will become an error in future releases, and are errors
                    by default when using the new API.
                    """,
                ]
                if self.allow_duplicate_keys is None:
                    warnings.warn(DuplicateKeyFutureWarning(*args))
                else:
                    raise DuplicateKeyError(*args)
            return False
        return True
return True if key is unique
dynaconf/vendor/ruamel/yaml/constructor.py