There is a plugin for Calibre that allows for many methods of finding duplicate books. The thread where discussion and development of this plugin occurs appears to be here. Thus, I believe the primary developer to be "kiwidude" on the mobileread.com forums.

However, I was unable to find a public git (or other version control) repository for this plugin, and I wanted to contribute further functionality. So, until I can figure out how the primary developer prefers to receive contributions (if at all), I've uploaded the entire plugin here, along with a commit for my change.