先上效果图

 template代码，新增数据时，点击出现弹框

<el-form-item label="分类名称"
                      prop="cateName">
          <el-input v-model="form.cateName"
                    placeholder="请输入分类名称" />

点击确定按钮，提交时作判断

submitForm: function () {
      this.$refs["form"].validate(valid => {
        if (valid) {
          // 判断
            //this.foodCateList是列表的数据
            //cateName是分类的字段
            //this.form.cateName是输入的值
          if (this.foodCateList.findIndex((item) => { return item.cateName == this.form.cateName }) == -1) {
            if (this.form.id != undefined && this.form.id != "" && this.form.id != null) {
              updateFoodCate(this.form).then(response => {
                this.$modal.msgSuccess("修改成功");
                this.open = false;
                this.getList();
              });
            } else {
              this.isSubmitForm = false;
              addFoodCate(this.form).then(response => {
                this.$modal.msgSuccess("新增成功");
                this.open = false;
                this.getList();
              });
            }
          } else {
            this.$message.warning('该分类已存在')
          }
        }
      });
    },