How does Klasku stop a teacher from being booked in two places at once?

A teacher's availability is judged on the PERSON, not on the class. Saving a schedule that would have one teacher in two overlapping sessions is refused, including when the two classes sit at different branches.

Room and class capacity clashes are checked at scheduling time as well, not when the teacher is already in front of the class. What is compared is the hours, so back-to-back classes may still be taught by the same person.