Indeed, it is an eventuality that can happen when we set up a computerized system. First come, first served! For all tests whose events are inseparable as clearly explained in the Information Bulletin: https://alcmontreal.com/wp-content/uploads/2020/05/Informations-inscription-TEF.pdf ., you must ensure that the choice of times for all the tests to be taken on the same day are still available, otherwise you must agree to choose another date. Only pay for the proofs that the calendar offers you.
For the TEFaQ tests , this is not a problem because you can take the tests individually. However, make sure you don't pay for 2 events for the same day if you can only take one! The online registration form allows you without any difficulty to go back and modify your choice of event for the day offering availability. You just have to open the part again: CHOOSE YOUR TEF/TEFAQ EXAM in order to modify.