Hi @robinhuang
I tested this myself and I figured post-login action also runs on Successful silent authentication. So if you login and your app also trigger a Silent Authentication check then that will be cause the post login to run again hence that action is triggered twice. You can check Actions triggered as part of login flows in Monitoring → Logs and select the Success Login event to see details.
Let me know if that is the reason in your case.
Thanks
Jeff