It's already happening. I'm getting robo calls from some police donation fund that will slow down and restart speaking if you interrupt them (took me a few moments to realize it was a recording).

Charities and political organizations are excluded unfortunately from robocall regulation. Google, though, is not.

If I were to start receiving Duplex calls, and could detect it, I would report each one to the FTC.

Perhaps they should identify themselves upfront? "Hello, this is an automated call on behalf of..."