For some weird reason, the feeds I get from quickanddirtytips.com (like Grammar Girl) behave strangely in the last several updates of DoggCatcher. They used to download latest episodes just fine, but now, when the latest one is downloaded, all the previous links in DC point to it too. I delete one and they all go away. Effing strange. Try it and see. And, no. The XML feed does not specify the same file name for each episode. I swear. And did I say that it used to work?
It sounds like you are having the same problem as described here - http://www.snoggdoggler.com/?q=node/88.
I added this to the bug list. Thanks for bringing it to my attention, this one seemed to slip through the cracks.