Whenever I'm DJing I always try to get there for the start of the night so I don't repeat anything that has been played, although it has happened on occasions. If you are being paid to DJ, you have to accept some responsibility in earning that wage. Simple as that.
Joan of course has hit the nail on the head with the two room scenario, it's impossible to be in two places at once, so really I'd spend most of the night in the main room, and trust to luck in the second room. theoretically, because you would play a different set in the second room, it should be individual enough for tracks not to be repeated.
Mind you, I've seen people follow me and start their set with a record I played less than ten minutes before, and I know they were in the venue then, so were they listening ??