added a early return when a double start is done. (cf issue 2)

src/listener-rss.ts

@@ -71,6 +71,8 @@ class ListenerRss extends EventEmitter {
    * @brief call the callback function each looptime
    */
   start(): void {
+    if (this.loopRunning) return;
+
     this.loopRunning = true;
 
     const fun: () => void = async () => {