code: static boolean hasDuplicateDeclarationSuppression(
        AbstractCompiler compiler, Node n, Node origVar) {
      // For VarCheck and VariableReferenceCheck, variables in externs do not generate duplicate
      // warnings.
      if (isExternNamespace(n)) {
        return true;
      }
      return TypeValidator.hasDuplicateDeclarationSuppression(compiler, origVar);
    }
docs: Returns true if duplication warnings are suppressed on either n or origVar.