How to create Duplicate Layer below the current layer in Photoshop?

If it is something you do a lot, then make an Action:

Cmd J

Cmd [

Unless you use a small screen, and need to toggle the major panels on and off to make space, I suspect most Photoshop users miss out on some useful single key shortcuts.  With the exception of F1 (Help) all of the Function keys can be used as single key triggers for your Action.  The only one I tend to use is F5, so it makes sense to me to re-purpose those keys to more useful functions.

And don't forget that Keyboard Shortcuts, along with Custom Toolbars and Menus can be saved with new Workspaces, so you can 'borrow' their default functions, and switch back something like Essentials if you need the regular shortcuts.  I actually use F6 and F7 to select and reset my Triple Monitor, and the Essentials Workspaces respectively.  I think most of us drag panels around our screens for convenience — this is especially so for regular posters on this forum who move panels to get screen shots for forum answers — so it is sooooooo nice to clean things up with a single key press.

https://forums.adobe.com/people/Bo+Peep  wrote Is there a way to duplicate a layer, but have it appear underneath the original?

no

you can move the layer down as a step after it has been made but what is the point... by definition they are duplicates

Hi there,

As of now there is no automate way of doing so, however you can record Action, then duplicate layer, and go to layers menu item, and click move down. Stop recording, and assign to shortcut key. please check this article to know more about actions.Actions and the Actions panel in Photoshop

Regards,
Sahil