How to refresh certain type of model when using ember-data?

ENV: Ember-1.8.1, Ember-data-1.0

I was trying to refresh some data in store. But I didn't build an adapter.

The models and relationships was like:

// user
{
    posts: DS.hasMany('post')
}

// post
{
    user: DS.belongsTo('user')
}

Something I did to reload data was like:

function reload(postPayload)
{
    // assume there are 8 records in postPayload
    console.log(user.get('posts')); // first time: 0 records, second time: 8

    store.unloadAll('post');

    console.log(user.get('posts')); // first time: 0,         second time: 0

    store.pushPayload(payload);

    console.log(user.get('posts')); // first time: 8,         second time: 16
    console.log(store.all('posts')); // first time: 8,        second time: 8
}

// this could be called many times
$.getJSON(url, reload);

The Problem here is when I try to get posts through user.hasMany relationship, the result doubled. But it seemed to be ok if I use store.all('posts') to filter posts.

Please help!