So yesterday I upgraded my Home Assistant setup from OpenZWave to Z-Wave JS. The upgrade itself was not very difficult. It was a bit time consuming as I had to remap the node names and ids to what they were before the migration. Not great, but not terrible either.
Things started going sideways when I looked into Google Home to see what devices where there. Each ZWave node appeared there 3 times. I searched in this forum and elsewhere but I did not find anything decisive about how to fix the problem. I did find some people saying they deleted their Home in Google Home and recreated it but I was wary of that solution. I tried unlinking Google Home from Home Assistant, relinking, etc., nothing worked.
So out of exasperation I deleted my Home and recreated it. Big mistake.
I recreated the house but all my devices where imported without being assigned any room. That baffled me because I thought the rooms were automatically created. I fiddled a while with that thing trying to put devices in rooms but I ran into a bunch of problems. One major issue was that the settings for the Google devices, i.e. the Google speakers and displays, were unreachable. I’m thinking now that these devices where still assigned to the home I had deleted and could be accessed only from that home. I deleted the new home I created and recreated a home again. This time the devices were in their proper rooms, but the settings for the Google devices were still unavailable. (I have no clue why the first time the devices were not in room and the second time they were. Given the code quality of Google Home, I’m pretty much ready to chalk it up to a bug.)
At that point, I had noticed that when I first deleted my home Google was saying something about “leaving the home” whereas now I was “deleting the home”. I figured my home probably still existed on my wife’s phone. I asked her for her phone and yes, the old home was there, and the devices were not duplicated. (I don’t know why.) So I deleted the home on my phone and send myself an invite from my wife’s phone giving me complete access. I have a few items missing, and a few nits to fix, but nothing major.
What I would have done differently?
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Check on the devices owned by other members of your family. The Home there may not have duplicate devices and you may just need to invite yourself. (I don’t know what happens when rights are asymmetric.)
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Rename the old Home and create a new one. That way the Google devices would still be accessible in the old home and I could move them to the new one. I screwed myself by deleting the old Home first. Fortunately, my wife’s phone saved me.
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If you’re single, consider creating another Google account and inviting it to your home. That way you have a backup of your home. I might just do this even if I can use my wife’s phone. I’ve accumulated a few Google accounts over the years.
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I’d really like to find a solution that does not involve deleting the home.
ETA: deleting the home caused even more problems that I detected at first. It also deleted my Nest home. 
I had to re-create my house and re-add all the smoke detectors and the door bell. Even after doing that Nest was telling me I had an unassociated subscription. I had to return into Google Home (of all places) and reassign the subscription to my house.
Conclusion: deleting a home to fix this is not a solution.