Can they then at least pick one scraping site from all the others (to prevent duplicate information)?

Although this is perfectly legal, there is quite a few big sites that scrapes Wikipedia and adds ad-sense. If you are searching for a narrow topic, page 1-2 is WP and WP copies.

As a sidenote - I see that Google Search has upvotes, comments and deletes in the search - yet it does not seem as if anyone uses it. Why? Why can't the user at least blacklist sites?