Problem with edge:"rising"

In another post about the LED and button tutorial, the following was stated:

setWatch can be called like setWatch(swap_on_down, BTN1,
{repeat:true,edge:"rising"}); so that it only calls your swap_on_down
function when the button becomes pressed.

I used that code but sometimes when I press the button once, my function gets called twice. I thought maybe I was being careless and tried very carefully to press and release it without any hesitation. I found that, no matter how careful I was, sometimes the function would get called once and sometimes 2-3 times. I changed the code to use "falling" instead and it seems to work reliably - the function only gets called once per button press.

Has anyone else experienced this behavior? Perhaps there is a bug? Is the switch debouncing implemented in hardware or software with Espruino?