I was reading this question and I am very surprised.
Does not installing a redirect from /index.php?qa=123&qa_1=why-do-bird-ssing to /123/why-do-birds-singh really make search engines consider the two pages as duplicate content?!
This is very surprising to me, because even MediaWiki (which even the very popular Wikipedia uses) allows both kinds of URLs to exist without redirects.
Duplicated content is ahuge blow to SEO. Do the different URL structures for the same content really get considered as duplicated content by popular search engines?