I am trying to make a shadow effect.
I put all the items I want in one layer called base.
Then I duplicated the layer and shifted it to the bottom and called it shadow.
I then hide the base layer to work on the shadow layer.
Any which way I try to move the the items in the shadow layer, the items in the base layer move instead.
Variations I have tried:
- instead of duplicating layer, create new layer and duplicate items then try and move them
- moving the shadow layer to the top to easier select the items
- instead of drawing a rubber band around the items to select them, select items individually or use the select all menu entry

What am I doing wrong?

Try this please,
Duplicate the layer (>Layers >Duplicate Current Layer) (Menu Bar not the Docked Shift+Ctrl+L),
use the >Layer >Lower Layer (Menu Bar)
Now use the Status Bar (bottom of inkscape UI) Layer Pulldown Menu and you may notice the PULLDOWN Menu is OUT of ORDER. To fix this just select each Layer then HIDE and LOCK them. Now the Layers Pull down should be in the CORRECT order.

Try to MINIMIZE using the DOCKED Layer menu (Shift+Ctrl+L) on the right as much as possible. IMHO there are still some kinks in that menu, where the Status Bar Pulldown menu and >Layer Menu (Menu Bar) options work better (for me anyway)...

To check if this worked Select the Shadow Layer (via pulldown in the Status Bar) and Unhide/Unlock it, hopefully all the objects will appear.

Now you can select all the objects in the layer Ctrl+A, and use the >Filters > Shadows and Glows >Drop Shadow... Menu to add the shadow effect.

If this does not work, How many objects are you working with?

If you've hidden the base layer, there should be no way to select items in it. Not without going to some trouble.
Are you using gradients at all? It may be if you duplicated a gradient, that's being used in both layers....but no, if it's hidden, it should be difficult to select it by accident. And even if you were able to select something from a hidden layer, once you moved that shadow layer above the base, that should solve the problem. This altogether sounds strange to me.

What Inkscape version and OS? Are you using a tablet?

Oh-oh-oh, I think I've got it -- if your objects are all subpaths of the same path, even if they are on different layers, they will all move together. Or if they are grouped.

I determined this quirk was because the items I duplicated had been modified using the 'Path Effect Editor'>'Envelope Deformation'. So I have decided to change my workflow and use that effect later in the process. This has brought about a different problem. Since this new prob is unrelated I started a new thread (please read).