
They require two separate grounds.  One ground goes to the ground pin
of the outlet, and the other ground wire is connected to the outlet's
mounting tabs (and thus grounds the box and faceplate screw and metal
faceplate, if any).

The box/faceplate ground goes to the normal distribution panel ground.
The outlet ground-pin wire is generally connected to an insulated
busbar in the distribution panel which is, in turn, connected to
the building-entrance main ground by its own wire.