@Saurabh I just test many orders again as conditions above, and found this:
When user tries to press COD 2 times (if it's loading slowly - may be caused by low internet speed), there will 2 orders comming to admin dashboard, one is correct, one is wrong.
Now I am trying to test again, and everything is right.
So I need more time to perform this issue again, if it's happening, I will tell you.
Thanks for your time!