20 Ga. Double-automatic would be quite a find and, so would a unicorn.
Never made them.
DoubleAuto = Steel Frame
Twelvette, Twentyweight = Alloy Frame
They made a "twentyweight" that was 3/4 of a pound lighter than the Twelvette. One barrel length 26.5 inches.
Browning Double Automatic, Twelvette, Twentyweight
"This model was manufactured between 1952 and 1971 in Belgium by Fabrique Nationale. There were three variations in 12 gauge only: Standard, Twelvette and Twentyweight. The alloy frame was available in different colors including royal or light blue, satin gray, green, gold, black and brown. Standard barrel lengths were 26, 26 1/2 (Twentyweight only), 28 or 30 inches with or without a vent rib."