I've read “The War of the Worlds” by Wells and currently am reading “1984” by Orwell. While reading I often met with constructions like:

must be within range of the Martians'' HeatRay now

The Parsons'' flat was bigger than Winston's

Mrs Parsons'' eyes flitted nervously from Winston to the children

I put my attention that after plural posessive forms of nouns (Parsons'', Martians'') two apostrophes stand. I used to think that there must be only one apostrophe. Is it right and why is it so?