validator addmethod username duplicates

1$("#signup").validate( {
2    rules: {
3        username: {
4            required: true,
5            minlength: 5,
6            remote: {
7                url: "dbquery.php",
8                type: "get",
9                data: {
10                    action: function () {
11                        return "checkusername";
12                    },
13                    username: function() {
14                        var username = $("#username").val();
15                        return username;
16                    }
17                }
18            }
19        }
20    },
21    messages: {
22        username: {
23            required: "Enter a username"
24        }
25    },
26    submitHandler: function(form) {
27        form.submit();
28    }
29});