"His pencils are black and red."
Translation:Pensilnya hitam dan merah.
Halo teman! Certainly, this is because plural nouns do not exist in Indonesian, so the same noun can be translated as singular or plural in English. The use of nominal reduplication is just used to stress the idea of the plural quantity, but it is not mandatory. This reduplication is often avoided in colloquial speech.
I hope it helps.
Selamat belajar! :)