If you and another user are updating the booking simultaneous, system will only allow 1 user to save the booking.
You will see on the top corner
If you hoover over the icons, you will see the name of the other user currently in the same booking
If other user is 'quicker' than you, system will give you a message that you're not in the latest version and you will not be able to save your changes
If you go to the versions, you will see the new version and you should proceed in that version.