It isn't that hard to scoop up all of the duckweed, then keep doing it on a regular schedule before another big invasion can build up. If you lose a few guppies, you will soon have a lot more anyway. I never had a problem trapping fish when I scooped duckweed, because the layer was so dense no fish could hide in it. After the first time I was dipping the stuff out daily for several days until there was no more to reproduce.
It wasn't a completely serious post, I was mostly whining. With one tank it's not so bad, with a dozen or so it's a bit of a nuisance. All the tanks got cross infected with the Python water changer. One tank today was just about done, then I noticed the filter was full, then a big clump of algae had a lot of DW caught in it. Progress continues.