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

You probably have an old version of the cgi package, which contained the above instance declaration before it was moved to cgi-undecidable. Hence the "duplicate instance declaration" message. Try updating your cgi package first.