There's a new tool available for Python programmers: Clone Digger. While it has not been officially released, it is available from the svn repository. Clone Digger finds code duplications in a given project, and creates a fairly comprehensive report (html file). Seeing the duplications on a screen is a powerful motivation for refactoring.
Check it out!
Just to make it clear: I had nothing to do with this project; I just found out about it via the gsoc-python-mentors list.