Renji was helpful enough to email me with a sample cache table with a duplicate, but I still haven't been able to reproduce this myself.
Some quick questions for you fine folks:
1. Are duplicates showing up on posts or pages?
2. Did updating the post (i.e. creating a new revision) trigger the duplicates?
3. Are you all really using Akismet? Does disabling Akismet prevent further duplicates from being created?