When Doris Duke, a fabulously wealthy tobacco and power company heiress, ran over and killed a longtime employee and confidant at her Newport, Rhode Island, mansion in 1966, police took her at her word that it was an accident.But the story of the killing at Duke’s Rough Point estate, which has resonated in the seaside tourist mecca, is being challenged by a witness — the paperboy.Bob Walker, a Marine Corps veteran, says he was there the day Duke killed 42-year-old Eduardo Tirella, driving into