fix #206: prevent 2 simultaneous background gathers

lib/session/call-session.js

@@ -449,6 +449,10 @@ class CallSession extends Emitter {
 
   async enableBotMode(gather, autoEnable) {
     try {
+      if (this.backgroundGatherTask) {
+        this.logger.info('CallSession:enableBotMode - bot mode currently enabled, stop it');
+        this.disableBotMode();
+      }
       const t = normalizeJambones(this.logger, [gather]);
       this.backgroundGatherTask = makeTask(this.logger, t[0]);
       this._bargeInEnabled = true;