Why is this form not submitted twice when hitting the button twice?

Question

Edit 25.07.2018: As Pawnesh Kumar said in the answer, this seems to be a browser issue. If I hit the button multiple times in Firefox the below script will only

Answer 1

1. How do you know that your form was not submitted twice or more times?

The fact, that you do not have multiply rows in database doesn't proof that. Maybe column "name" is unique in your table?

2. Check logs of your webserver. Example for apache: sudo cat /var/log/apache2/access.log

There you will find info of how many request you recieved.