$output doubled after ajax add to cart

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	public function callback() {
		$this->language->load('module/cart');

		$this->load->model('tool/seo_url');
		
		unset($this->session->data['shipping_methods']);
		unset($this->session->data['shipping_method']);
		unset($this->session->data['payment_methods']);
		unset($this->session->data['payment_method']);	
		
		if ($this->request->server['REQUEST_METHOD'] == 'POST') {
			
			if (isset($this->request->post['remove'])) {
	    		$result = explode('_', $this->request->post['remove']);
          		$this->cart->remove(trim($result[1]));
      		} else {
				if (isset($this->request->post['option'])) {
					$option = $this->request->post['option'];
				} else {
					$option = array();	
				}
				
      			$this->cart->add($this->request->post['product_id'], $this->request->post['quantity'], $option);
			}
		}
				
		$output = '';
		
		if ($this->cart->getProducts()) {
		
    		foreach ($this->cart->getProducts() as $product) {
        		
				$output .= '<a href="index.php?route=checkout/cart">' . $this->cart->countProducts() . ' ' . $this->language->get('text_shoppingbag') . '</a>';
				
	            }
			
    		
    		$total = 0;
			$taxes = $this->cart->getTaxes();
			 
			$this->load->model('checkout/extension');
			
			$sort_order = array(); 
			
			$view = HTTP_SERVER . 'index.php?route=checkout/cart';
			$checkout = HTTPS_SERVER . 'index.php?route=checkout/shipping';
			
			$results = $this->model_checkout_extension->getExtensions('total');
			
			foreach ($results as $key => $value) {
				$sort_order[$key] = $this->config->get($value['key'] . '_sort_order');
			}
			
			array_multisort($sort_order, SORT_ASC, $results);
			
			foreach ($results as $result) {
				$this->load->model('total/' . $result['key']);

				$this->{'model_total_' . $result['key']}->getTotal($total_data, $total, $taxes);
			}
			
			$sort_order = array(); 
		  
			foreach ($total_data as $key => $value) {
      			$sort_order[$key] = $value['sort_order'];
    		}

    		array_multisort($sort_order, SORT_ASC, $total_data);
    	    		
          		foreach ($total_data as $total) {
      			
      			   if ($total['title'] != 'Sub-Total:') {
            $output .= '<div class="product_total">' . $total['text'] . '</div>';
        }

      			      		}
      	
		} else {
			$output .= '<div style="text-align: center;">' . $this->language->get('text_empty') . '</div>';
		}
		
		$this->response->setOutput($output, $this->config->get('config_compression'));
	} 	
}
?>