The festival usually takes this into consideration and tries to schedule multiple screenings of a film. (The same does not apply to sessions.) In case you are unable to book a particular film screening, you can do one of the following:
Refer to your screening schedule to see an alternative screening date, day and time and book it in advance.
Check online for sudden availability due to cancellations.
Arrive at the screening auditorium much before the screening time to check if the rush line/ walk-in seats are available at the venue or if there have been any cancellations.