There are a few ways to duplicate a layer: You can drag it to the Create a New Layer icon at the bottom of the layers panel; you can press Command-J (PC: Ctrl-J)—my favorite method; or, you can hold down the Option (PC: Alt) key and drag out a copy in the Layers panel. Maybe you knew all of these, and maybe you didn’t. But, did you know you can also duplicate multiple layers at the same time? Click the top layer you want to duplicate in the Layers panel, and then scroll all the way to the last layer you want to include. Shift-click that layer, and all the layers will now be selected. (You can also Command-click [PC: Ctrl-click] noncontiguous layers to select them.) Now, use any of the aforementioned methods, and all the selected layers will be duplicated at once.
This tip previously published in Colin Smith’s Photoshop Tips column, in the March, 2020 issue of Photoshop User magazine.