Usually the duplicate answers happen in unison. If it's an easy question, there are usually around 5 answers posted simultaneously. If this happens, you're usually supposed to delete your answer if you see a better one, but as you can imagine there are different criteria used to determine which answer is better. Ultimately this is solved by the voting and answer selection by the asker.
That answer is wrong too because Beautiful Soup also exists for Ruby: http://www.crummy.com/software/RubyfulSoup/
If you read the Rubyful Soup web site you would've noticed that it's unmaintained and the author recommends hpricot.
Apparently the "right" answer to this language-agnostic question is one that provides a single Ruby (surprise) alternative. Since when do opinions have correct answers?
I would love to see great things from Stack Overflow, but I would be surprised if it turns out to provide better answers than can be found by googling and reading mailing list archives -- or at least better answers that are consistently marked as such (or not marked at all, depending on the situation). Introducing voting doesn't magically mean the "good" stuff floats to the top.
Edit: Oh, and people are already "trolling for reputation" as evidenced by a single question having 10 of the same exact answer. I thought this is what voting was going to "solve?" sigh