The first person to sign up for a slot will have the slot locked for ten (10) minutes. After ten minutes it is possible that someone could take the slot, although until someone else does the original person can still confirm. Once the slot is confirmed, they can’t lose the slot unless they cancel the slot or an Administrator cancels them.
We also have a feature called auto-confirm. By default, once someone has confirmed one time, we place a cookie (a small packed of information) into their browser that says this person is confirmed. The next slot they go to sign up for can then be auto-confirmed, meaning that they get the slot right away without having to get a confirmation email again, assuming they signed up for the second slot on the same computer and browser as the first.