http://darcs.haskell.org/packages/cgi-undecidable/

There only 3 or 4 lines of source code, but it can't be installed.

The ghc version is 6.6 .

Undecidable.hs:23:0:
    Duplicate instance declarations:
      instance [overlap ok] (MonadTrans t, MonadCGI m, Monad (t m)) =>
                            MonadCGI (t m)
        -- Defined at Undecidable.hs :23:0
      instance [overlap ok] (MonadTrans t, MonadCGI m, Monad (t m)) =>
                            MonadCGI (t m)
        -- Defined in Network.CGI.Monad