Why is this event being handled twice in knockout?

For some reason the below code makes the links' click event fire twice. I'm relatively new to knockout and I think I might have done my custom binding wrong. Can anyone tell me what I've done wrong? (Btw, the reason I didn't post a fiddle is that I can't include the mapping plugin from github on jsfiddle.)

JS:

ko.bindingHandlers.activityContent = {
    init: function(element, valueAccessor, allBindings, viewModel, bindingContext) {
        // This will be called when the binding is first applied to an element
        // Set up any initial state, event handlers, etc. here
        var content = document.createElement("p");
        content.innerHTML = '<a href="javascript:void(0)" data-bind="text: user_name, click: $parent.NavigatePage.bind($data, \'profile\', user_id)"></a>';
        element.appendChild(content);
        ko.applyBindings(bindingContext, content);
    },
    update: function(element, valueAccessor, allBindings, viewModel, bindingContext) {
        // This will be called once when the binding is first applied to an element,
        // and again whenever the associated observable changes value.
        // Update the DOM element based on the supplied values here.
    }
};

var activities = ko.mapping.fromJS({Activities: [{
    "user_id": "52b5042d572b94ceadf6asdf1a2a5bc",
    "user_name": "Sean Templeton"
}, {
    "user_id": "52b5042d57asfda2b94ce61a2a5bc",
    "user_name": "Sean Templeton"
}, {
    "user_id": "52b5042d572b94ce61a2a5bc",
    "user_name": "Sean Templeton"
}, {
    "user_id": "52b5042d5asdfasdf72b94ce61a2a5bc",
    "user_name": "Sean Templeton"
}, {
    "user_id": "52basdf5042d572b94ce6asdf1a2a5bc",
    "user_name": "Sean Templeton"
}], NavigatePage: function(page, userId) { console.log(this); console.log(page); console.log(userId()); }});

ko.applyBindings(activities);

html:

<Ul data-bind="foreach: Activities">
    <li data-bind="activityContent: $data"></li>
</ul>

Remove the ko.applyBindings(bindingContext, content); line in your init, and remember that update runs once on init, and then on each update.

Also, you have a larger problem. In the fiddle given above by @nemesv, by not removing the ko.applyBindings() from the custom binding, the list isn't even iterated the full 5 times.

A DIFFERENT APPROACH

Unless you have great plans for the activityContent binding beyond this example, you could dispense with the custom binding and simply register a delegate event on your <li> element in your viewModel. You could then use Knockout's ko.dataFor() to determine what's been clicked on, and then call NavigatePage(). This approach is far more performant and does not require a custom binding.

You can read about delegate events here.