Sounds like a great way to spam and gain link pop without the mods noticing it.
Good catch! The HTML is still loaded, even though the text is not shown. Parsers will always read it - humans may not.
Quotes would be a good idea for this, but then some less than honest individuals would modify the quotes.
The best solution is coming - let machine learning insert spoiler tags (or some other name) when the content is duplicated, and draw attention to it when there are modifications.