A dual check is two check valves in a single body, with no shutoff valves and no test cocks. It is small, it is inexpensive, and on a great many residential services it is sitting at the meter without the homeowner knowing it is there.
It is not a double check assembly
This is the one thing worth getting right on this page, because the names are nearly identical and the two devices are not interchangeable.
A double check valve assembly — a DCVA — has shutoffs and test cocks, is certified annually, and is approved for low-hazard commercial connections. A dual check has none of that. Where a provider's cross-connection control program calls for a DCVA, a dual check does not satisfy it, and the difference is not something that can be argued about in the field. If somebody has told you your property has "a double check," it is worth knowing which of the two is actually on the line.
What it does
Two checks in series give a basic barrier against both backsiphonage and backpressure on a low-hazard residential service. Many water providers require one on every new residential connection as blanket containment — protection for the street, not for the house.
That last distinction matters. A dual check at the meter protects the public main from your property. It does nothing to protect your own plumbing from a cross-connection inside it, which is what the protection at hose bibbs and on the irrigation system is for.
It is not testable
No test cocks, nothing to attach a gauge to, no annual certification. A dual check is verified by inspection and replaced when it fails.
The common failure is not dramatic. A dual check that has stopped sealing does not announce itself — it simply is not doing anything, and it will keep not doing anything until somebody looks. They are also a frequent cause of the thermal expansion complaints that show up after one is installed, because a closed system has nowhere to put the extra volume when the water heater fires.
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