"Duplicate keys found during update" when add slide to array

[mukiwu]

I have a "add Slide" button to add slide to pages array

add() {
  pages.push({
    title: 'abc',
    ..
  })
}

<carousel v-model="currentSlide" :autoplay="autoPlayTime" :pauseAutoplayOnHover="true" :mouseDrag="false" :wrap-around="true">
  <slide v-for="item in pages" :key="item.title">
     {{ item }}
  </slide>
</carousel>

but when I add it, console will get warn: [Vue warn]: Duplicate keys found during update: "title" Make sure keys are unique.