The ominous “Doomsday” plane has been spotted flying over California — for the second time in two months. The huge Boeing E-6B Mercury, which is the president’s emergency operations hub if nuclear war breaks out, coasted over Fresno over the weekend. Airport bosses said it was conducting approaches at the site and spent two hours doing mock landings, leaving onlookers stunned. The aircraft serves as a nuclear control platform and strategic command post. If the plane had to execute its mission,