IDL compiler doesn't check if union case labels are unique which leads to not
compiling C++.

From corba spec:
Name scoping rules require that the element declarators in a particular
union be unique.

invalid IDL that is accepted:
 enum ValueSort2 {
    SYMBOL_TYPE2
  };
  union NonRecursiveUnionWithStringStruct switch (ValueSort2) {
  case SYMBOL_TYPE2:
    Symbol2 symbol_val;
  case SYMBOL_TYPE2:
    Symbol2 symbol_val2;
  };