Yeah, not every instance of reduplication (repeated word) signifies a plural. Tricky, right? If you reduplicate an adjective, it's for emphasis or to increase the attribute. /Cepat/ means 'fast, quick'. /Cepat-cepat/ can be 'really fast' or 'more quickly'.
Likewise with /oleh-oleh/. This is not a plural, technically, even though it uses a pattern that would make a noun plural. It's a phrase meaning "souvenir" but specifically the kind of souvenir that is eaten (and often bought as a gift for other people by the traveler).