return false not working with mvc button

@using (Html.BeginForm("Add", "EmailTemplate", FormMethod.Post, new { id = "submitForm", enctype = "multipart/form-data",onsubmit = "javascript:return fnDuplicatecheck()"}))
        {

<input type="submit" value="Save" class="float-r btn-gray1" />
}


javascript



function fnDuplicatecheck() {
        loader();
        debugger;
        var TemplateName = document.getElementById('TemplateName').value;
        $.ajax({
            url: '@Url.Action("GetDuplicateTemplate", "EmailTemplate")',
            type: "GET",
            data: { TemplateName: TemplateName },
            success: function (data) {
                debugger;
                if (data == "Yes") {
                    $("#popup_ok").click();
                    jAlert("Template Name Already Exist", "Error");
                    return false;
                }
                else {
                    $("#popup_ok").click();
                    return false;
                }
            }
        });
    }