As Nevelle has suggested, I have set up two Protege counters on my wheel. This practice is something I highly recommend. The cost is negligible, especially when calcilated against the opportunity costs associated with measuring a specific course.
The double readout gives me a great sense of confidence as the measurement is proceeding, and makes it unlilely that a count will be missed.
Actually, if the magnet and sensor are carefully placed, I think it more likely that a spurious count will be added, rather than having one missed.
As one is moving along a course, it is not uncommon to have to stop for one reason or another, and if the wheel comes to a stop with the magnet too close to the sensor, it may trigger an extra pulse.