code: private void removeDuplicateDeclarations(Node externs, Node root) {
      Callback tickler = new ScopeTicklingCallback();
      ScopeCreator scopeCreator =
          new Es6SyntacticScopeCreator(compiler, new DuplicateDeclarationHandler());
      NodeTraversal t = new NodeTraversal(compiler, tickler, scopeCreator);
      t.traverseRoots(externs, root);
    }
docs: Remove duplicate VAR declarations.