如何在XML中定义多次出现

<students>
  <student>
    <name>John</name>
    <age>20</age>
  </student>
  <student>
    <name>Jane</name>
    <age>22</age>
  </student>
</students>

<books>
  <book id="1" title="Book 1" author="Author 1" />
  <book id="2" title="Book 2" author="Author 2" />
</books>

<orders>
  <order>
    <id>1</id>
    <customer>Customer 1</customer>
    <items>
      <item>Item 1</item>
      <item>Item 2</item>
    </items>
  </order>
  <order>
    <id>2</id>
    <customer>Customer 2</customer>
    <items>
      <item>Item 3</item>
      <item>Item 4</item>
    </items>
  </order>
</orders>