I had a magnetic reed switch connected to an emonTH for my gas meter, that counted approximately 2.5 per pulse. The addition of a 0.1 µF capacitor across the reed switch completely eliminated the problem. I suggested adding a note about this to the Wiki in January - it’s not happened.
I also have an optical pulse sensor, but that’s connected to an emonPi; so not directly relevant. That has on occasions picked up a nearby fluorescent lamp (with a thermal starter) being switched on or off. Others have found light getting to the sensor has been counted (perhaps obvious in hindsight).