1. You can ducplicate layers in the LAYERS menu >Layers >Duplicate Current Layer (it is not clear in your post if this is what you used to duplicate your layer)
then use the Layer Menu to Lower it below the original creation >Layer >Lower Layer (Shift+Ctrl+PgDn)
2. You can also create sub-layers, create a New Layer (Shift+Ctrl+N), use the pull down menu to set the POSITION "As sublayer to current layer"
1&2. Then you could apply an opacity value to the ENTIRE LAYER (sublayer) using the Layers (Shift+Ctrl+L) docked menu's Opacity Slider/Number Entry (located at the bottom)http://tavmjong.free.fr/INKSCAPE/MANUAL ... ayers.html
3. In th FILTERS menu there is a Drop Shadow Menu...
>FIlters >Shadows and Glow >Drop Shadow... [Drop Shadow Menu]
The Drop Shadow menu has these options.
> Blur Radius,
> Opacity %
> Horiz. Offset
> Vertical, Offset
4. If you are going to work with UI and Icons, I suggest you read up on cloning as this feature is used in inkscape's shared icon files. There is are various ICON .svg files in the /inkscape/share/icons folder. To be safe copy a file and save it under a different directory and name. I've manipulated the pattern file and they no longer work right so be careful in this SHARE directory.http://tavmjong.free.fr/INKSCAPE/MANUAL ... lones.html
your choice, Draw Freely!