RadioGroup fires 'change' twice

http://stackoverflow.com/questions/9229471/extjs-4-radiogroup-change-event-fires-twice

HTML

<link rel="stylesheet" href="http://dev.sencha.com/deploy/ext-4.0.7-gpl/resources/css/ext-all.css">

JavaScript

var gr = Ext.create('Ext.form.RadioGroup', {
    id: 'WheatChoice',
    padding: '',
    layout: {
        
        type: 'hbox'
    },
    fieldLabel: 'Choose Model',
   
    allowBlank: false,
    columns: 1,
    flex: 1,
   
    items: [
        {
        xtype: 'radiofield',
        id: 'SpringWheat',
        
        fieldLabel: '',
        boxLabel: 'Spring',
        name: 'wheat-selection',
        inputValue: '0',
        flex: 1},
    {
        xtype: 'radiofield',
        id: 'WinterWheat',
       
        fieldLabel: '',
        boxLabel: 'Winter',
        name: 'wheat-selection',
        inputValue: '1',
        checked: true,
        flex: 1}
    ]
});
gr.on('change', function() {
    Ext.getBody().insertHtml('afterEnd', '<div>changed ' + (new Date()).getTime() + '</div>');
});
gr.render(Ext.getBody());