Is it keys should be globally unique?

function Book(props) {
  const index = (
    <ul>
      {props.pages.map((page) => (
        <li key={page.id}>{page.title}</li>
      ))}
    </ul>
  );
  const content = props.pages.map((page) => (
    <div key={page.id}>
      <h3>{page.title}</h3>
      <p>{page.content}</p>
      <p>{page.pageNumber}</p>
    </div>
  ));
  return (
    <div>
      {index}
      <hr />
      {content}
    </div>
  );
}