If I use Bruxish's "Synchronoise" attack against my opponent's dual type Pokemon like Volcanion EX, and there are other Volcanion EX on the Bench, do they take 40 damage or 20?
They take 20 damage.
Source: Burning Shadows FAQ (2017-07-20), TPCi Rules Team (2017-08-03)

Relevant cards

Note: All applicable cards may not be listed below.
SM3_38