I believe they refer to duckweed as the herpes of the fish world on another planted tank forum, so take from that what you will.
It is painful to have to do any sort of maintenance on a tank with duckweed. One of mine has the whole surface covered so it is always stuck to my buckets, siphon, arm and once I found a glass in our cupboard with duckweed on it!
Really you are stuck with it, unless you can find a way to push it to the side while you work. I only tolerate it as it looks nice and is a massive nutrient sponge for my heavily stocked tank.