WordPress in Directory Redirect Help

We have our long time blog with settings
wordpress url – http://www.site.com/archives
site url – http://www.site.com/archives

All urls are like
http://www.site.com/archives/2011/07/postname

Based partially on hosting wordpress in directory
http://codex.wordpress.org/Giving_WordPress_Its_Own_Directory
we added a custom index.php file to make http://www.site.com/ the main page.

Now there is duplication of content at
http://www.site.com/
http://www.site.com/archives

Q: How can we redirect only the the single front page of
http://www.site.com/archives
to
http://www.site.com/

while keeping rest of the urls unchanged with /archives/