I got mine at my LFS. They said hey you can have this clean healthy duckweed if you buy this bichir. So the compulsive shopper that I am, I bought the bichir and got the duckweed. It was nice for awhile, but it took over my tank and like you said, there's no getting rid of it. I cleared it all out (so I thought) and then in two weeks time, my tank was covered in them yet again. They are cool looking, but more of an annoyance plant. Unless you seriously want it, I wouldn't advise. I was told that I would end up throwing out handfuls. Skeptical at first, I said whatever, but sure enough I just threw out two handfuls from my other tank last week and more buds are coming in..
I kind of wanted to make a 'swamp' look for my frog aquarium, for that reason I am kind of tempted.
I've not seen duckweed around here, I saw some growing in a little creek that may of been duckweed but I'm not adventurous enough to go down and check it out yet.
Hey at least you probably have great water quality. I hear duckweed does have a highly positive effect on water. : )