A diaphragm pump (also called a Membrane pump) is a positive displacement pump that runs on the combination of the reciprocating action of a Air Diaphragm Pump rubber, thermoplastic or teflon diaphragm and suitable valves on either aspect of the diaphragm
A dual diaphragm is 
a positive displacement pump which utilises two flexible diaphragms that reciprocate back and forth, creating a short-term chamber, which both allures and expels fluid through the pump. … Both diaphragms that are linked by a shaft through the center section where in fact the air valve is located.
Though it depends upon the pump and application, a positive displacement pump, is normally a self-priming pump. … For example, air operated diaphragm pumps self-primary by creating a pressure differential in the diaphragm chamber. This draws in air flow and pulls the fluid into the suction port.