def validate_selector_default(selector_file: SelectorFile) -> None:
    """Check if a selector.yml file has more than 1 default key set to true"""
    default_set: bool = False
    default_selector_name: Union[str, None] = None

    for selector in selector_file.selectors:
        if selector.default is True and default_set is False:
            default_set = True
            default_selector_name = selector.name
            continue
        if selector.default is True and default_set is True:
            raise DbtSelectorsError(
                "Error when parsing the selector file. "
                "Found multiple selectors with `default: true`:"
                f"{default_selector_name} and {selector.name}"
            )
Check if a selector.yml file has more than 1 default key set to true
core/dbt/config/selectors.py