Metaplural

N. 1.) A plural word that is derived from a word which itself is pluralized.

Adj. 2.) That which is a plural of a plural; like a metaplural.

"Peoples" is a metaplural (that means more than one group of people, usually of different nationalities) because "peoples" is the plural of "people", and "people" is the plural of "person".
Similarly, "fishes" is a metaplural; you use "fish" as a plural when referring to only one species, but if you have multiple species, you use "fishes", plural of "fish" (which is the plural of itself).