Forbid to define duplicated member types in Union (#266)

Author: IvanGoncharov

spec/Section 3 -- Type System.md

@@ -752,7 +752,7 @@ Union types have the potential to be invalid if incorrectly defined.
 1. The member types of a Union type must all be Object base types;
    Scalar, Interface and Union types may not be member types of a Union.
    Similarly, wrapping types may not be member types of a Union.
-2. A Union type must define one or more member types.
+2. A Union type must define one or more unique member types.
 
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