#121 add_event_detect callback is called 2 times per detect

[Adam]

See this thread for details https://www.raspberrypi.org/forums/viewtopic.php?f=28&t=131440. While using any kind of button or switch, even with hardware debounce, I get 2 callbacks per button press/switch. In the thread you can see I tried joan's pigpio module using the same circuit and pigpio works as expected.

[Ben Croston]

See also issue 124 and issue 79