by Julieanne Kost
With the Move tool selected, Option (Mac) / Alt (Win) -dragging a layer in the image area creates a copy of the selected layer(s).
You can also Option (Mac) / Alt (Win) -drag in the Layers panel to duplicate a layer – select the layer(s) and drag (up or down) until yo see a heavy black line between the layers and release.
Command + J (Mac) | Control + J (Win) will also duplicate the selected layer(s). Note: this shortcut works for Layer Groups as well.