When preceded by a numeral or quantifier, plurality is not normally marked on the noun, although it is certainly not unacceptable. Count nouns with nonspecific reference also optionally appear with the plural suffix. The reduplication of nouns involves a pluralizing semantic effect; this is the only context where plural marking appears to be unacceptable.
Source: Kouwenberg 2007: 441
Source: Kouwenberg 2007: 445
Source: Kouwenberg 2007: 445
Source: Kouwenberg 2007: 445
Source: Kouwenberg 1994: 239