Insert input only if it does not already exist

I solved your problem the following way, although I think that to resolve otherwise.

I added to checkbox an attibuto data-add that will guarantee if the field was inserted or not, it was this way.

JQuery

$(function(){
    $('#add-button').click(function(){
            $('.check').each(function(){
                if($(this).is(':checked'))
                    if(!$(this).data('add')) {
                        $('#form-receiver fieldset').append("<p>" +$(this).parent().text()+ "<input name='" + $(this).parent().text().toLowerCase() + "'></p>");
                        $(this).attr('data-add', true);
                    }
            });
        });
});

You can track the result in DEMO

If you do not want to use attributes data- you can do this using a variable to control it.

var checked = [];

$(function(){
    $('#add-button').click(function(){
            $('.check').each(function(index){
                if($(this).is(':checked'))
                    if(!checked[index]) {
                        $('#form-receiver fieldset').append("<p>" +$(this).parent().text()+ "<input name='" + $(this).parent().text().toLowerCase() + "'></p>");
                        checked[index] = true;
                    }
            });
        });
});

You can track the result also in this DEMO     

In fact, although there are already good answers, the error in its solution and lack of concatenation in the check selector:

// adicionei os operadores de concatenação de string '+'
if(!$('#form-receiver').has("input[name='" + $(this).parent().text().toLowerCase() + "']")

And also the .has() method of jQuery does not return a boolean , but the result of a selection, so you would have to check if someone returned the selector:

$('#form-receiver fieldset').has("input[name='" + $(this).parent().text().toLowerCase().trim() + "']").length == 0

And I would not recommend creating tags name with whitespace, I'd make a trim() :

$(this).parent().text().toLowerCase().trim();

Track your working solution here .

But I still recommend using the @Diego Vieira solution for being cleaner.

I would avoid using names with spaces. So I suggest a function to make CamelCase source here :

function camelCase(input) {
    return input.toLowerCase().replace(/[-\s](.)/g, function (match, group1) {
        return group1.toUpperCase();
    });
}

Then you could use this code:

$('#add-button').click(function () {
    $('.check').each(function () {
        if (this.checked) {
            var nome = $(this).parent().text();
            if (!$('#form-receiver').find("input[name='" + camelCase(nome) + "']").length) {
                $('#form-receiver fieldset').append("<p>" + nome + "<input name='" + camelCase(nome) + "'></p>")
            }
        }
    })
});

Example: link

In what way the only relevant difference between my code and yours is that I used it   if (!$('#form-receiver').find( .... ).length . The length is only as a precaution, but the find is to look for the element within the form. I also changed to this.checked because you do not need jQuery to do this check.