def _add_essential_scope_source(
        self, scope_source: Union[ScopeSource, List[ScopeSource]]
    ) -> None:
        """
        Add essential scope sources to self._essential_scope_sources.
        When self.validate() is called, we make sure that all source info are not missing.
        """
        if not isinstance(scope_source, list):
            scope_source = [scope_source]

        for source in scope_source:
            if source in self._essential_scope_sources:
                raise ValueError(f"{source} already exist in _essential_scope_sources.")
            self._essential_scope_sources.append(source)

        Add essential scope sources to self._essential_scope_sources.
        When self.validate() is called, we make sure that all source info are not missing.
        
coremltools/converters/mil/mil/block.py