Now, person is singular, and can decline to the plural persons or people, depending on region, level of formality, and nuance. Let's ignore persons for the moment and just focus on people. People, when used as a singular collective noun, can further decline to the plural peoples. For example, I could say that I am a person from Gillikin Country, which has many people, and all of them are part of the peoples of Oz.
Now, my question is this: are there any other words that behave this way? That is to say, are there any other words which have a singular, plural, and collective plural inflection?