(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?
Halo teman saya! I have had conversations with Indonesians from West Java and Bali since 2012 when I started to study music of gamelan in Buenos Aires; my teacher is from Jogya. Then, Indonesian, Javanese and Balinese became very interesting languages for me and I started to read some grammar and texts, to watch films and listen to music every day.
I remember this example of plural nouns with sentences for animals: Kucing adalah binatang, Anjing dan kucing adalah hewan. Then, when I asked if I could use the duplication, a native Indonesian just started to laugh because hewan-hewan and binatang-binatang do not sound natural. But I still think this duplication can be used sometimes, perhaps if the subject is definite, for example: The monkeys in the temple, the street dogs in Bali (I think the duplication can be used in these cases.) And the use of beberapa can described an indefinite number: I find some/many birds in Borneo, I have some rescued dogs. If the number is little then we can use sedikit (some/few or a little) before the noun.
Some example sentences:
”Anda mengatakan Anda mencari kucing?” ”Benar. Saya mencari kucing-kucing yang hilang."
Benarkah orang pintar hanya memiliki sedikit teman?
Since I cannot determine if all the nouns can avoid the use of duplication to describe a plural subject or object, I use a little rule: if the pronoun (kita, kami, kalian, mereka) is describing the subjet and object then I do not use duplication, if I use general verbs as Saya cinta, Saya punya, Saya suka, then I do not use duplication. Only if I need to add information to describe the number of animals or things, then I think it is better to use beberapa or banyak (many) before the noun.
There are some phrase as Semoga sukses! meaning Good luck! or Best wishes! We need to understand the noun is used as a mass noun. Do not worry if in other languages the noun can be singular. For example, in Spanish we use: ¡Mucha suerte!, ¡Que tengas/tengan mucho éxito!, ¡Muchos éxitos! or ¡Mucho éxito! These phrases are used invariably.
Semoga sukses untuk bahasa Indonesia! =)