When someone schedules a meeting with the same start time as another scheduled meeting, the system should warn the person that there is another scheduled meeting at that same start time, but it does not.
It only tells you that there is another meeting in session when you attempt to actually start your meeting and the other one is already active.
@Cjeff The reason there is no warning is because you're allowed to schedule overlapping meetings in order to set up different events for Co-Organizers to host themselves. When there is a need for simultaneous sessions, multi-user plans are available to upgrade. You can even manage groups of users by enabling or disabling their assigned services through the Admin Center.