Ember: dumplicated records are created in store after model save action

Question

I have two models in my ember app:

App.Order = DS.Model.extend  
  orderItems: DS.hasMany('orderItem')  


App.OrderItem = DS.Model.extend
  order: DS.belongsTo('order')

I tried to save orderItem, bellow is the payload I got after save invoking.

{
  orderItem:{
    id:1
    order:"1"
  },
  orders:[
    {
      id:"1",
      orderItems:["1","2","3"]
    }
  ],
  orderItems:[
    {
      id:"1",
      order:"1"
    },
    {
      id:"2",
      order:"1"
    },
    {
      id:"3",
      order:"1"
    }
  ]
}

Then, I found 2 duplicated orderItems records in ember store with same id(id=1). I don't know how does ember push payload after model saving. error will be shown if removing orderItem(id=1) from orderItems hash.

Who can tell me what's the root cause?