fix bug: set scheduler max_instances=3

server.py

@@ -54,7 +54,9 @@ async def add_task(task_args):
     if scheduler.get_job(task_name):
         scheduler.remove_job(task_name)
     trigger = IntervalTrigger(seconds=interval)
-    scheduler.add_job(task_handler, trigger, args=[task_args], id=task_name, replace_existing=True)
+    scheduler.add_job(task_handler, trigger, args=[task_args], 
+                      id=task_name, replace_existing=True,
+                      max_instances=3)
 
 async def list_task():
     jobs = scheduler.get_jobs()