Duplex pump is a pump used in oil fields to pump the mud from the surface to the well-bore through the Drill String. It consists of two pistons. It has higher flow rate than the single pump because of not having a dead spot in the pump stroke. However, it is considered to be less efficient than the triplex pump.
As one piston is finishing its stroke cycle, the other one would start its stroke. Working by this mechanism, it can keep large flow rate of pumping without a break. The below figure shows the formula used to calculate the flow rate for this pump.