You can see a few slight differences in the pictures in the other topic - https://lovehoneyforum.com/t/if-the-insideout-didnt-work-for-me-is-it-worth-giving-the-duo-a-shot-at-all/275074/3
I think a lot of the price difference is made up of the extra features, like battery size, Smart Silence and Autopilot, etc. You can check out the comparison table too for a bit of an overview. 