Best answer by melvimbe
Unfortunately, there isn't a solution currently. When speaking to an echo (or some other device), alexa doesn't currently associate a specific sonos room to the device, and doesn't know where ducking needs to occur. It's definitely something on Sonos radar and something they are addressing with Amazon.
There are two exceptions to this. The first is as you noted, TV audio won't duck. The second is if your alexa device is a sonos beam or Sonos One. In this case, sonos obviously knows what room you're in and will only duck that room.