As some of our band mates think that they are on tour with Genesis, we only play parties / functions where the client has seen us and that are booked well in advance, so we have a contract / booking form - this is essentially a checklist to say we will need a few things like floorspace, a set up time and sound check and a running order of the evening, which also allows the client to ask a few questions. to date its also avoided double bookings.
Years ago in a different band we were once double booked at a pub, we split the set up so the other band played, we played and then we did a big jam at the end - making the best of bad job, good fun too.