"In case a conflict is found, the behavior depends on the setting "Force when overlap" in the Add multiple slots screen. If this is set to "no", then those new slots that would create a conflict will not be created. If the switch is set to "yes", then existing conflicting slots will be deleted before the new slot is being created.
However, Scheduler will never delete a conflicting slot if students are already booked on it, or if the slot is located outside the current scheduler activity."
But even if "Force when overlap" is set to "yes" when adding multiple sessions, the conflicting previous sessions are deleted. It seems a kind of bug. Any ideas how to workaround this situation?