I very rarely sneeze just once, usually there's two coming in a row, and I've noticed the same thing in other people and even in animals. Is there any reason for this?
The very simple answer is that the causal factor or trigger hasn't disappeared after the first sneeze. Sneezing is a reflex, partially autonomous, to clear the nasal cavity of particles that don't belong there. As long as the trigger is not removed, the reflex is repeated.
This is also one of the reasons that allergies that cause sneezing, such as hay fever, continue as long as the factor causing the allergy is present.