Hello
no, Scheduler doesn't currently do this (and was never designed to do so). If you set the scheduler to allow "only one appointment", then it allows exactly this - one booking per student.
One option might be to switch to "one appointment at a time". Of course, then in principle every student could book another appointment once you have marked them as "seen"; I'm not sure whether that matters in your case.
Another workaround is to delete the previous appointment of the student, which will allow them to book another one.