def METHOD_NAME(self, node, deep=False):
    """Create mapping while checking for duplicate keys."""
    mapping = set()
    for key_node, value_node in node.value:
        key = self.construct_object(key_node, deep=deep)
        if key in mapping:
            raise ValueError(
                f"Backfill entry already exists with entry date: {key}."
            )
        mapping.add(key)
    return super().METHOD_NAME(node, deep)