[Haskell-cafe] What's wrong with "cgi-undecidable"?

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