Key callback is being fired twice

[losnir]

Given the following code snippet:

var data = {
    name: 'Scott',
    children: [
        { id : 1, name : 'Alice' },
        { id : 2, name : 'Bob' },
        { id : 3, name : 'Chris' },
        { id : 4, name : 'Robert' }
    ]
}

var mapping = {
    'children': {
        key: function(data) {
            console.log(data.id);
            return ko.utils.unwrapObservable(data.id);
        }
    }
}
var viewModel = ko.mapping.fromJS(data, mapping);

Which is taken from the documentation, with slight modifications.

Expected result in console would be: 1 2 3 4

Actual result is: 1 2 3 4 1 2 3 4.

jsFiddle: http://jsfiddle.net/losnir/a2qMQ/

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As mentioned in the title, the key callback is being fired twice.

The culprit seems to be on line 570 (filterArrayByKey, 1st fire) and on line 582 (keyCallback(unwrappedRootObject[i]), 2nd fire).