I'm not sure. I can't reproduce it every time, but it happens many times each play session.
I have a MacBook Air, Mid 2011, running OS X 10.9.1, and unity 4.3.4f1.
I'm going to guess that this has to do with the framerate, actually. If I can get one click in within a frame, both down and up will trigger.
I also use tapping the touchpad, not clicking. This might also reduce the time.