Proposed resolution for #21: duplicateAttributes

"Some people want to ban this, but no algorithm is proposed yet." is not correct, since  
GNF normalization easily detects such duplicate attributes.  On the other hand, GNF 
normalization is often more expensive than validation.

Here is my propsal: This is not an error.  Implementations MAY issue error messages.

Cheers,

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MURATA Makoto