[Haskell-cafe] duplicate instance declarations. Why?

with:

{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}

class A a class R a

class S a

instance R a => A a instance S a => A a

GHC gives *Duplicate instance declarations* * instance R a => A a * * instance S a => A a * ** *Why?*