If you possess multiple instances (may be different versions, may be same version different builds) of a package, life can be hard and confusing. The problems are explained in my http://www.vex.net/~trebla/haskell/sicp.xhtml and faced by many people regularly. (Just read this mailing list.) cabal-install has a mechanism for not adding more instances. It is available since version 0.14.0, or earlier. It is just little known. It is also a bit manual. You have to give 1 instruction for each package. If you want to say it for n packages, you have to give n instructions. Also, if you give such an instruction for a package you do not already have, there is a problem: now you can't install that package. To insure package sanity, add these lines to your $HOME/.cabal/config, one line per package you want to protect. (Cannot merge into one line.) constraint: array installed constraint: bytestring installed constraint: Cabal installed constraint: containers installed ... Generally, do it for every package that comes with GHC, and every package that comes with the Haskell Platform if you have it, plus every package that you want stable. (You may omit packages that cannot possibly be on hackage, e.g., integer-gmp.) Remember to revise those lines whenever you switch GHC versions or Haskell Platform versions, since their package lists change. Alternatively, if you want even more manual work (in exchange for more fine-tuning perhaps?): every time you use "cabal install": cabal install --constraint="array installed" --constraint="bytestring installed" --constraint="Cabal installed" --constraint="containers installed" ...