Can layers be cloned? If so, how? I ask because although I can see a thread with a relevant title at alpha.inkscape.org/vectors/www.inkscapeforum.com/viewtopic081e.html, I didn't notice an answer to the thread's title within it, nor any evident option in Inkscape's GUI (see the attached screenshot). I'm using inkscape-1.4-2.fc41.x86_64.

Are you sure that you have opened Inkscape 1.4, even if it is installed ? 
Go to Help>About Inkscape and check.

I'm using Inkscape 1.4 (1:1.4+202410161351+e7c3feb100)

You do however have to left click the layer to select it, before right clicking to see the duplicate / clone options.

You can also left click and use Edit>Clone.

If you do not left click first to select you will only get the options you see in your screenshot.

1. Are you sure that you have opened Inkscape 1.4, even if it is installed?

   Yes, for the relevant window provides the undermentioned:

   Inkscape 1.4 (e7c3feb100, 2024-10-09)

2. If you do not left click first to select you will only get the options you see in your screenshot.

   That works! Thank you. Do you know why the options differ?

I'm not 100% sure, it does seem it may be a bug ? 

Thanks. I've filed gitlab.com/inkscape/inbox/-/issues/11467.

This is likely related to my report: https://gitlab.com/inkscape/inkscape/-/issues/5324