Using Mulitple on post select shows multiple selections on edit

I have a post select field on one of my pages that uses the `multiple` option.

```php
array(
'id' => 'home_featured_pages',
'name' => __('Featured Pages', 'edit2017'),
'type' => 'post_select',
'use_ajax' => true,
'multiple' => true,
'query' => array(
'post_type' => 'page'
)
)
```

When I edit the page to begin with it works as it should letting me select as many pages as I want.

After saving the page and coming back to edit it I get each page in its own select2 box.


All the boxes have the same id/name and only the first one gets saved.

I'm using the latest `dev-master` code.