I believe that you meant to say double check instead of duel check valve. Two way different horses.
Mick
I guessing that you are pointing out that "official" plumbing terminology tries to differentiate between the device known as a "duel check" (which is a device that consists of just two check valves back-to-back) and a "double check backflow preventer" (which is a device that consists of two check valves back-to-back between two shutoff valves, with test cock beteween these components allowing the operation of the two check valves to be tested independantly of each other).
But in some circles, words "duel check" and "double check" improperly get used interchangably (tomato / tamoto).
I initially learned most of what I know about irrigation at
www.irrigationtutorials.com, and the irrigation egineer there tries to explain the difference between "duel" and "double". IMHO, his text is a bit confusing. So I try to focus on the test cocks to differentiate between the two devices rather than on a name.