[Vuejs]-Vue js drag and drop how to remove duplicate id in array

new Vue({
  el: "#demo",
  data() {
    return {
      categories: [
        {id: 0, title: "This is parent block", children: [{ id: 0, title: "AAA", categoryId: 0 }, { id: 1, title: "BBB", categoryId: 0 },],},
        {id: 1, title: "This is parent block", children: [{ id: 2, title: "CCC", categoryId: 1 }, { id: 3, title: "DDD", categoryId: 1 },],},
      ],
    };
  },
  methods: {
    onDrop(e, categoryId) {
      const itemId = parseInt(e.dataTransfer.getData("itemId"));
      const id = categoryId === 0 ? 1 : 0
      const child = this.categories[id].children.find(c => c.id === itemId)
      child.categoryId = categoryId;
      this.removeFromList(id, itemId)
      this.addToList(categoryId, child)
    },
    addToList(categoryId, child) {
      this.categories[categoryId].children = [...this.categories[categoryId].children, child];
    },
    removeFromList(id, itemId) {
      this.categories[id].children = this.categories[id].children.filter(c => c.id !== itemId);
    },
    onDragStart(e, item) {
      e.dataTransfer.dropEffect = "move";
      e.dataTransfer.effectAllowed = "move";
      e.dataTransfer.setData("itemId", item.id.toString());
    },
  },
})

.droppable {
  padding: 15px;
  border-radius: 5px;
  background: #2c3e50;
  margin-bottom: 10px;
}
.droppable h4 {
  color: white;
}
.draggable {
  background: white;
  padding: 5px;
  border-radius: 5px;
  margin-bottom: 5px;
}
.draggable h5 {
  margin: 0;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.5.17/vue.js"></script>
<div id="demo">
  <div
    v-for="category in categories"
    :key="category.id"
    @drop="onDrop($event, category.id)"
    class="droppable"
    @dragover.prevent
    @dragenter.prevent
  >
    <h4>{{ category.title }}</h4>
    <div class="child">
      <div
        v-for="item in category.children.filter(
          (x) => x.categoryId === category.id
        )"
        :key="item.id"
        @dragstart="onDragStart($event, item)"
        class="draggable"
        draggable="true"
      >
        <h5>{{ item.title }}</h5>
      </div>
    </div>
  </div>
  {{categories}}
</div>