Thank you, that's useful info. I think I didn't explain the case well enough though.
Basically, in most cases, it's okay for the pages to only have 1 parent and only appear in 1 list on 1 page.
However, some pages need to be listed on multiple parent pages.
Something like this:
Home
---- Horror movies
--------- A horror movie
--------- Some horror/thriller movie
---- Thriller movies
--------- A thriller movie
--------- Some horror/thriller movie
So let's say Some horror/thriller movie has Horror movies as its parent. How can I make it appear on the Thriller movies page as well, along with that pages other children?
Maybe you already answered this, but do you have a concrete example?