Receptional_Andy - 4:19 pm on Jul 5, 2010 (gmt 0)
This certainly shouldn't happen if there is indeed a permanent redirect from one to the other. I would verify that Google is indeed receiving a 301 status - you could use the tool in GWT to "fetch as googlebot", for instance.
Another possibility is that Google is merely manipulating the display URL in SERPs, rather than actually storing a distinct copy. This seems to happen when Google anticipates that a redirected URL will get more clickthroughs for a particular search.
One way to check if this is happening is to use the search syntax below:
In cases where Google has correctly mapped redirects, you'll usually see the canonical URL returned for that query. In those instances, it's a display issue rather than a problem that would affect performance.