There may be a way to do this, though the obvious step of implementing a 301 redirect in .htaccess won’t work because it’ll create an infinite loop.
However, rather than try to do the redirect, I would recommend using the Canonical URL link element, see
http://www.google.com/support/webmasters/bin/answer.py?answer=139394
This is the perfect situation for use of the canonical element, because resolving the duplicate content with a 301 is particularly difficult.
In this thread:
http://modxcms.com/forums/index.php/topic,44222.0.html Ryan gave this code for the Revo version of the canonical element:
<link rel="canonical" href="[[++site_url]][[*id:isnot=`[[++site_start]]`:then=`[[~[[*id]]]]`]]" />
The other thing is to be careful never to link to anything other than the proper friendly URL for a page.
This should resolve any trouble with duplicates, including ones you may not have thought of.