While working on and trying to install py-yt, Spack installed py-scipy as dependency. When the install of py-yt failed, I corrected its package, and tried to install it again. Spack then began by re-installing py-scipy. I don't know why -- nothing in this package on its dependencies changed, and none of their variants changed either. However, the hash is different.

I still have the respective prefix directories of py-scipy if that helps.

I've seen this problem several times with several different packages; currently, Spack is re-installing py-scikit.