We can simply duplicate a layer using the layer palette. Drag and drop the layer which needs to copy to the "Create a New Layer" button. It will generate a duplicate copy of that layer.
Or this can also be done by using the menu. Layer > Duplicate Layer... This will take you to a dialog box called Duplicate Layer.
You can rename the new layer using this dialog box. You also able to select the destination. Destination means you can copy this duplicating layer into any of the opened creation (document) or as a new creation. Select the document in the Destination area of the dialog box.
Layer via Copy is an option to duplicate a selection of a desired layer. This selection can be done by using the marquee tools. If there is no such selection the entire layer will duplicate on this option. This can be done by using the menu Layer > New > Layer via Copy or using the shortcut key Ctrl+J.