Reduplicated nouns refer to individual occurrences of the item denoted by the noun (giving a distributive sense), in contrast with regular plurals, formed by suffixing -apu, which may refer to accumulations of an object. Reduplicated nouns are not attested with the plural suffix. The marginal status of noun reduplication is also seen in the fact that elicitation of reduplicated nouns proved impossible: Speakers invariably rejected them.
Source: Kouwenberg 2007: 445
Source: Kouwenberg 1994: 245