With a Group targeted in the layers panel, Select Layer > Duplicate Group or Option (Mac) / Alt (Win) -drag the Group in the Layers panel (until you see a heavy black line between the layers) and release.

Note this shortcut will not work if the Group is at the top of the Layers panel - in this case, I use the context sensitive menu (Control -click (Mac) or right mouse click) on the Group and select Duplicate Group.