Why is the plural sometimes indicated by duplicating the noun when other times the singular suffices?
It depends on the context.
Reduplication is not always necessary to indicate the plural form.
For example in this sentence, it's not strange to assume that you need more than one book to fill a house.
It's obvious (implied) that it's plural, so it's not necessary to use the reduplication.
The context indicates the plural form.
It's not wrong to use it here in this sentence, but it's not necessary.
Where it's not correct to reduplicate is when the quantity is mentioned, like this :
dua buku-buku <==that is incorrect.
sepuluh buku-buku <==that is incorrect.