Im using this Plug-In.
Newly imported Files will be hashed while they are imported. Simply make sure that "search for Duplicates" is checked in the import dialog. Already existing Images must be hashed extra, with "Tools -> Hash for Duplicates"
In F-Spot 0.5. (Jaunty) this feature is broken, in 0.6.1.3 (Karmic) it works, but it doesn't give a lot of feedback.