I found that (SE blog) Announcing The Launch Of Meta Stack Exchange and (SO blog) Announcing The Launch Of Meta Stack Exchange are absolutely the same, right down to the comments. No redirect, just duplication. Why?
It's the exact same blog post (and the exact same blog), just available at two different URLs.
In fact, if you look at the HTML source for either of the pages, you'll find the tag:
That tag is there to tell search engines that the official URL of this blog post is the one with the host name blog.stackoverflow.com; the blog.stackexchange.com address is just an alias.