(My opinion) "Tidak efektif" (not effective) means yeah just like what you say, it's not good if you use "mereka,kalian,kami" then you pluralized the "petani". Just like in english, the plural of "farmer" can't be "farmer-farmer", but "farmers". In english, we need to add "s/es" to the last word. But in Indonesia, we would't add "s" to the last word, just say "petani". Maybe sometimes we pluralized "petani" into "Beberapa petani", but it doesn't make sense in this case. Hope this make sense :D
I agree with you Diego. You are a lot more advanced than me in Indonesian, I've just started it 2 months ago, but I noticed too, that plural is tacit, implied in Indonesian, when shown by another word, like "mereka". The doble words to show plural is not needed anymore in this case, but I don't think it's wrong (or Duolingo should revise most of its lessons), you can use the doble plural-word, but it's less natural. I think they should accept It here, as it accepted everywhere else, or almost.
Indonesian people always said in the comments that anyword-anyword is always less natural than "beberapa anyword" when "beberapa" is possible (or banyak)
But they should accept it.
How long have you studying Indo Diego?