phpmailer exception in Gravity form

phpmailer exception in Gravity form

I have involved with a Mailer error. I am using gravity forms and there is
my form. I submitted to form and a email had delivered successful. but i
got error everytime
Fatal error: Cannot redeclare class phpmailerException in
/home/content...../class-phpmailer.php on line 2825
i have used also:-
if (!class_exists('PHPMailer')) {
require_once('class.phpmailer.php');
}
Don not know why i am getting. please help me