Starting a second session for the same license plate in the same zone is allowed in some municipalities, which makes it possible to start a new session before the end of the previous one as an alternative to extend.
If you have accidentally started a second session, you can easily stop the second session. This prevents a full payment of the parking period.
On the main screen of the PayByPhone service you will see all running parking sessions. If there is one that is not listed, it is because it did not start.