title: Laravel orderBy back up field incase primary is duplicate
body: <p>Working on a queuing system that allows users to book appointments.</p>
  
  <p>There is a slight issue at the moment where two appointments can get the same value for a field of 'estimated_booking_time'. This time is used to show when they are coming up.</p>
  
  <p>The problem is that when fetching all appointments the ones with a duplicate estimated_booking_time are then ordered by their ID which (depending on when the appointment was booked) can sometimes be in a reverse order.</p>
  
  <p>Im using carbon to create a diffInMinutes return on another field but the where the ID is the backup order my results are not always correct.</p>
  
  <p>Is there a way to tell Laravel instead to use another field as the backup orderby field when there are duplicate values?</p>