Unless you want to always keep duckweed, I still recommending avoiding it. There are far better plants that can be left floating to diffuse light if that is what is wanted.
I personally don't do floating plants except when first establishing a tank, or when culturing blackworms or in a no-substrate fry grow-out tank. The plants I use for floating are those that can also be planted, so that when their purpose is no longer needed as a floating plant, I can repurpose them as a planted plant. Floating plants, for what I like to do, defeat the purpose of putting on decent light to grow plants that are planted in the substrate.
I've personally had duckweed absolutely kill every plant in a planted tank when not kept under control. An 8day neglect (not taking out 80% of it daily) left nothing BUT duckweed in the tank, and it was so thick that even the topmost duckweed was killing off the duckweed under it.