I just installed several sonos speakers throughout the house.
When I'm in the living room, Google assistant in the bedroom responds.
I found out that the sonos with highest volume set, is the speaker which will respond.
I lowered the volume in the bedroom and the speaker in the living room started responding, instead of the bedroom.
I was experiencing the same issue with my Sonos One: when prompting Google Assistant, she would answer on both my phone (Google Pixel 3) and the closest speaker.
It was extremely bothersome as I hadn't had any trouble with this happening the first several months after installation. If the phone ever did register what I was saying and pull up the Google Assistant overlay screen, it would always simply recognize the conflict and defer to the speaker, sometimes displaying "answering on another device."
I could not figure out what had changed. Eventually, though, I realized the default Google account on my phone had changed from the one that I had registered the assistant with on the speaker. I don't know if it happened during an update, if I just fat-fingered it at some point, or if I had really only added it that recently.
IN ANY CASE: I just switched the default Google account on my phone back to the account that the assistant was registered under on my Sonos app and the issue was resolved. Can't guarantee this is the same for anyone else, but it worked for me.
TL;DR: Check to make sure the default account on your phone/your phone's assistant is the same as the one you registered on your Sonos app.
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