Two groups turn up for the same court
A league match is arranged by the captain, club night by the committee, and both land on Court 1 on Tuesday evening. The booking sheet on the clubhouse door did not stop it, because both were written on different copies of it.
Make each court a sub-calendar that refuses overlapping events. Whoever books second is told at once, with the booking it collides with, and can move to a free court while there is still time to tell people.
Refused on Court 1, booked on Court 3 instead.