I have extensively tested this. Remove my wife’s account removes all the double entities. Re-adding her account makes all the double entities reappear.
It seems like this is a bug in Home Assistant, but I can’t be 100% sure. I’ll report a bug.
Only one account should have your home assistant installation linked. The other account will still be able to access the devices although some things they won’t.
Sure, understood. Only my account has home assistant linked.
If you look at this logically. If they get duplicated when you add her account, then the devices exist in her account. They shouldn’t. You must have HA or the actual source devices linked in her account. 
Can you show screenshots of the link services from BOTH accounts/phones (after her account is removed from the home).
So I just changed my wife’s permissions to viewer which removed all the duplicate devices. Everything seems to be working. Not sure if this was a google bug, hassio or myself. It’s working so that’s great!
It’s strange - I have my wife’s account set as a viewer but I still get double entities for everything exposed by HA. If I don’t add/link HA to her google home account she has no access to some entities (but does have to others). Any thoughts what can be the issue?
I’m still having the issue, once I removed her, she was able to see the devices but she is unable to do voice controls. I added her back, but still haven’t found a solution
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So I removed my wife and looked at it again. She can control all the devices but I’m not able to have her access scripts. I think a work around will be creating a switch that toggles a scripts. Not sure if that is the problems you are having, but at least for me it seems to be scripts
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