[Haskell-cafe] duplicate instance declarations. Why?

[Alberto G. Corona]

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?*
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