reCAPTCHA: why can't I check the same result twice?

Question

I mean if I check first clients input and it is OK the second check of the same input is always false... Why is that?

I really need ability to check it twice (one for client side validation and second for server side validation)

Thanks in advance!!!

EDIT

Clarifying:

If user's input is ok and recaptcha returns true (I do it through ajax to my server which sends request to recaptcha's server) the form is submitting and sends via POST also 2 variables: recaptcha_challenge_field value and recaptcha_response_field value (which was already checked) and than my server asks recaptcha's server to check again this two values to do server side validation.

Jquery code:

$("#form_id").find("button").click(function(){
    var c = $("#recaptcha_challenge_field").val(),
        r = $("#recaptcha_response_field").val();

    $.ajax({
        url: "/ajax/captcha?challenge=" + c + "&response=" + r,
        dataType: "json",
        success: function(data){
            if(data['is_valid']){
                $.ajax({
                    url: "/ajax/captcha?challenge=" + c + "&response=" + r,
                    dataType: "json",
                    success: function(data){
                        if(data['is_valid']){
                            alert('OK');
                        }else{
                            alert('FAILED');
                        }
                    }
                });
            }else{
                Recaptcha.reload();
            }
        }
    });
    return false;
});

So, as you can see there are two absolutely identical operations with different result (it alerts only FAILED).

Answer 1

Because it is stored in a session that is cleared when the result is submitted. On page load, a new session variable for that CAPTCHA value is generated.