In the meantime, I've had help from a guy from comp.lang.python who
tested in depth and made some interesting suggestions which were
incorporated into the code. The whole code is now also fully functional
on Windows (Python is about being cross-platform, really).
The new version is 0.10, still available at
http://www.homepages.lu/pu/fdups.html. If you happened to download 0.08
or earlier, please be advised that they are buggy. Version 0.09 is
correct, but version 0.10 has a few new options.
Comments and ideas are still welcome.