A double valve device for the prevention of backflow and method of its use Abstract A double device (1) and method serves for mounting in a flow channel of a sewer (47) for the prevention of backflow, which sewer (47) has an intake pipe (50) and an outfall pipe (51) for sewage and/or water. The double valve device (1) comprises an elongated hollow valve body (2, 3) that includes a first elongate hollow member (2) defining a first channel (2a) and a second elongate hollow member (3) defining a second channel (3a) opposite the first channel (2a), an actuator (28) operatively connected to the elongate hollow valve body (2, 3) to allow adjusting the distance between the first elongated hollow member (2) and the second elongate hollow member (3) upon actuation, wherein the first elongate hollow member (2) has a first flap valve (20) pivotally between a closed state and an open state of a first valve opening (55) of the first elongate hollow member (2). The second elongate hollow member (3) has a second flap valve (27) pivotally between a closed state and an open state of a second valve opening (57) of the second elongate hollow member (3), which second valve opening (57) faces the first valve opening (55). The valve device is a backflow blocker, which, during daily operation, allows wastewater to pass by, but in damming and flooding situations automatically closes the flap valves in the drainage line so that water cannot flow into the drain upstream the valve device.