"Saya punya bebek-bebek dan gajah-gajah." Is not accepted. Reported, but i won't hold my breath.
How can you tell from the Indonesian sentence that "bebek" and "gajah" are respectively plural?
You can not. Unless you say "Saya punya bebek-bebek dan gajah-gajah"
But it would be redundant in daily conversation. That's why usually Indonesian people will follow with the question "how many of them do you have?" If they need to...