The majority of dogs have a dewclaw on the front legs but those with dewclaws on the hind legs are rare.
Double dewclaws refer to the extra digits on the hind legs.
The digits are not exactly digits they're more like a genetic anomaly in most cases. Dewclaws on the forelegs are fully developed toes.
A dogs legs have a bone structure as well as muscles and nerves that connect them. Alternatively the double dewclaws on the hind legs are often just a flap of skin with a claw sticking out.
Since they do not possess connective muscle tissue they are completely useless. However only a few breeds have fully-formed and functional double dewclaws.
Their condition is no longer considered a genetic anomaly.
It is actually a breed standard to have an extra toe on the hind leg.