I recently heard a new word on the radio: cherubims. The person who used it was probably unaware that “cherubim” is already a plural. It's the plural of “cherub”.
The mistake was understandable. The “-im” plural ending is from a language unrelated to English, and is rare in English. We English speakers have become used to the fact that the way to form a plural is to add “s”.
It wasn't always so. In Old English, the “s” ending was only one way to form a plural. A great many nouns formed their plural by adding things like “-en”. There are multiple reasons why the “s” plural eventually came to dominate, but I would assign the biggest part of the blame to the influence of Norman French. During the period of Norman rule in England, the people who spoke only English were mostly illiterate, so the French speakers had a huge influence on things like written documents. In terms of grammatical structure, modern English possibly has more similarities to French than to old Anglic or Saxon, and this includes the use of the French “s” plural.
That's an oversimplification, of course. Some English dialects already had a tendency to use the “s” plural more than the other kinds of plural, and if left undisturbed this tendency might well have spread to other regions. Even so, the Norman influence would have hurried that development.
Anyway, back to our muttons. I was talking about double plurals.
Perhaps the best example of a double plural is the word “children”. At one stage the English word for “child” was “cild”, and its plural was variously “cild”, “cildra”, or “cildru”. (The addition of the “h” was another Norman innovation. Scribes tried to impose French spelling rules on English words, ignoring the fact that English already had a perfectly good letter – the “c” – to indicate what we now call the “ch” sound.) There must have been some people who didn't recognise that “cildra” was plural, and tacked on a “-en” to form the plural. That gave us a word that contains two plural markers.
“Brethren” is another example where the “-en” ending was tacked on to a word that was already plural.
In modern English, interesting things are happening to the word “you”. We used to have two separate words: “thou” for addressing a single person, and “you” for addressing more than one person. Then, in feudal times, several European languages including English developed a distinction between “the familiar singular” and “the polite singular”. (But only for the second person singular pronoun.) In English this was done by using “you” (or, actually, “ye”, but that form has since fallen out of use) as both the plural pronoun and the polite singular. People would have said “thou” to their social equals, and “you” to people of higher rank.
This familiar/polite distinction still exists in languages like Italian and French and German. English overshot; in the long run, we abandoned the familiar form and kept only the polite form. We still know about the word “thou”, but we never use it except when consciously using archaic language.
[I'm perhaps being presumptuous with that “we”. My own ancestors spoke only Gaelic a few generations ago, and I have little or no genetic connection with the speakers of, for example, mediaeval English. On the other hand, we get our linguistic traditions through education, not through genetic descent.]
This leaves us with an ambiguity, because the word “you” can be singular or plural. Some people are obviously uncomfortable about this, because they have invented the plural “yous”. (Often written, illogically, as “youse”, despite the fact that it doesn't rhyme with “house”. No wonder that people complain that English spelling is difficult.) Although this form is generally considered to be substandard, I'm inclined to think of it as legitimate dialect variation.
It's possible to go one step further. On more than one occasion I've heard people using “yous” as the singular pronoun, with the plural “youses”. I'm not willing to predict whether this will ever become respectable.
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