Response should only be sent once.

flight/net/Response.php

@@ -29,6 +29,11 @@ class Response {
      */
     protected $body;
 
+    /**
+     * @var bool If a response has already been sent
+     */
+    protected static $sent = false;
+
     /**
      * @var array HTTP status codes
      */
@@ -169,7 +174,6 @@ class Response {
      * @return object Self reference
      */
     public function sendHeaders() {
-        if (!headers_sent()) {
         // Send status code header
         if (strpos(php_sapi_name(), 'cgi') !== false) {
             header(
@@ -204,7 +208,6 @@ class Response {
                 header($field.': '.$value);
             }
         }
-        }
 
         return $this;
     }
@@ -219,10 +222,16 @@ class Response {
             ob_end_clean();
         }
 
+        if (!self::$sent) {
+            if (!headers_sent()) {
                 $this->sendHeaders();
+            }
 
             echo $this->body;
 
+            self::$sent = true;
+        }
+
         return $this;
     }
 }