An Error has occurred

Am_Exception_InputError: This product is either not exist or you already have it. in /home/puzzlebookcompil/public_html/members/application/cart/library/Am/Widget/CartProduct.php:23
Stack trace:
#0 /home/puzzlebookcompil/public_html/members/library/Am/Widget.php(38): Am_Widget_CartProduct->prepare(Object(Am_View))
#1 /home/puzzlebookcompil/public_html/members/application/cart/controllers/IndexController.php(110): Am_Widget->render(Object(Am_View))
#2 /home/puzzlebookcompil/public_html/members/library/Am/Mvc/Controller.php(130): Cart_IndexController->productAction()
#3 /home/puzzlebookcompil/public_html/members/library/Am/Mvc/Controller.php(107): Am_Mvc_Controller->_runAction('productAction')
#4 /home/puzzlebookcompil/public_html/members/library/Zend/Controller/Dispatcher/Standard.php(307): Am_Mvc_Controller->dispatch('productAction')
#5 /home/puzzlebookcompil/public_html/members/library/Zend/Controller/Front.php(956): Zend_Controller_Dispatcher_Standard->dispatch(Object(Am_Mvc_Request), Object(Am_Mvc_Response))
#6 /home/puzzlebookcompil/public_html/members/library/Am/App.php(1430): Zend_Controller_Front->dispatch()
#7 /home/puzzlebookcompil/public_html/members/index.php(92): Am_App->run()
#8 {main}:This product is either not exist or you already have it.