In my college it is not unusual for some rooms to have multiple simultaneous bookings.

The most common example is when a subject like french has an AS class and an IB class, they are both taught by the same teacher, in the same room, at the same time. They are 2 distinct groups as far as the MIS is concerned so that exam entry etc. doesn't become a nightmare. The bookings themselves get into the database via my timetable import script (CONTRIB-1189)

This patch only works on the display side of things (using this patch will not allow you any new ways to bypass the validation). It displays simultaneous bookings in a different colour (configurable in the css as TD.DoubleBooked) and prints the details of both rather than overriding with the last booking (the default behaviour)