LR wants to re-add all my keywords as duplicates

I think we'd need to see a screen recording (not a phone video) to understand the details of what's happening:

https://www.descript.com/blog/article/how-to-screen-record-on-windows

Sounds like you are exporting photos (with keywords) and then adding these to the catalog again. If you do that, then make sure you export with the keywords in Lightroom's keyword hierarchy. If you don't check that option, then the keywords end up as new keywords in the root.

Thanks, but I'm not trying to add new keywords. I'm tagging my photos with existing keywords (e.g. the names of people I already trained LR's face recognition algo on). Again, it's not merely people tags. The recent/suggested keywords too, though clearly taken from my existing hierarchical set, get added to the 'root' of the hierarchy. That's the problem.

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"like tagging people through face recognition"

To tell LR where you want new person keywords added to the hierarchy, right-click the parent keyword, e.g. "People", and do Put New Keywords Inside This Keyword:

That keyword will be marked with a * to the right of it.