[Haskell-cafe] installation problem with cabal

Hi, when I install packages from cabal, the package manager installs already installed files (some of his favorites are process and haskell98) so these packages are both system and user installed (that can cause later dependency problems, but the cabal FAQ explains very well how to recover from it) but the funniest is that if I reinstall it, it again reinstall the same packages even they are already twice installed does anyone know how to solve this issue? Thanks Louis