jQuery validation plugin, check arrays

I want to use "jQuery validation plugin" (http://jqueryvalidation.org/) with arrays.

I tried this solution:

Validating array inputs using jquery validation plugin

But it's not working for me: only one element per array is being check.

My HTML is:

<form id="signupForm" method="get" action="">
    <fieldset>          
        <p>
            <label for="username">Username</label>
            <input name="username[]" type="text">
        </p>
        <p>
            <label for="username">Username</label>
            <input name="username[]" type="text">
        </p>
        <p>
            <input class="submit" type="submit" value="Submit">
        </p>
    </fieldset>
</form>

And my jQuery is:

$("#signupForm").validate({
    rules: {
        'username[]': "required"
    },
    messages: {
        'username[]': {
            required: "Please enter a username"
        }
    }
});

Here a jsFiddle: https://jsfiddle.net/t14ft1ny/