Importing RAW + JPG into LR CC results in duplicates

At this point in the development cycle, Lightroom simply isn't as highly developed with "advanced" features as Lightroom Classic is. Whether or not the features you are seeking are in the planning is something I don't know as I'm not privy to any of Adobe's future plans.

Alright, pardon me for mixing-up the terminology and feature names. What I meant was the former, treating sidecars of the raw as such. If that is how it was designed, then I would consider it to be a design bug. When we are expected to move to the cloud-based Lightroom while it misses fundamental design choices, it should be a bug. I beleive that there is already a similar feature request that has been open for almost three years with no attention from the dev team or adobe.

I think you're slightly mis-remembering the Classic functionality. There isn't an option to automatically have the imported Raw+Jpeg images stacked (though there's a post-import process to have images stacked by capture time, which could be used for this purpose). There is an option which you can set in the Classic preferences which gives the user the choice of two behaviours when importing Raw+Jpeg "pairs" (the option is to "Treat JPEG files next to raw files as separate photos"). If that option is enabled, the behaviour works exactly the same as the Lightroom desktop app, i.e. two separate files from each "pair". If the option is not enabled, Classic will copy both Raw and Jpeg to the destination folder, but the Jpeg half is only recorded as a "sidecar" file to the Raw file, it cannot be accessed by the user in any way (though if the raw file is renamed, moved or deleted, the same action is applied to the Jpeg sidecar). This is not the same as "stacking".

The Lightroom desktop does not have the "sidecar" option, so both Raw and Jpeg "pairs" are always imported as two separate photos. This is not a bug, it is as designed....though of course you could have your request converted to a feature request.