@ergative@JeremyMallin "The way I was taught" was that if the word itself is a singular ending in 's' then it gets an apostrophe and an 's'. Like "Venables's" in my example
If however the word is itself a plural like "peoples" ending in 's' then in your example then it simply has the trailing apostrophe to indicate a plurality, so "peoples<apostrophe>.
I shall find the cite when I get home, or at least an amusing rant about the subject 😂