I agree with this post. My fav. combination is the one with the black shadow, and blue highlight, and because we can customize the colors individually, replacing the blue with another color, while keeping the black shadow in place, also produces some incredible results.

There is a limitation though. If your Cover's background image is "fixed" the duotone does not work. My workaround for that is to layout a temporary Cover block, set the duotone, render it on the front end, and with the Windows Snipping Tool I capture the duotone image as a jpg. Now I can create my actual Cover block, use the captured duotone jpg image as my background, and simply set it to be "fixed". I have not tested it, but by doing so, we might even get better page load times, especially if the page contains too many duotone images.

Many (including me) often criticize Gutenberg for it's deficiencies, but when it comes to the Cover Block, the duotones, along with the Matrix control are top notch, It's time for me to give the devs. all the props they rightfully deserve in this case, what's fair is fair !