Identifying extra triggers in a speedometer The HRM, GPS, and cadence in my cyclocomputer give perfectly good readings, but the speedometer (and hence the odometer) readings are obviously incorrect.
Because the speed and distance readings seem to be just about twice what they should be, I'm guessing that something other than the dedicated magnet is triggering the extra events.

Might the Presta valve be magnetized and provide that extraneous trigger? Since the cadence and wheel receiver are in the same piece, there is not a lot of flexibility in moving the dual-receiver far away from the valve.
Have you been there? What are some ideas to debug this issue?
The problem went away after a firmware update. The most likely interpretation is that the reed switch is generating two pulses for each pass of the magnet, but now each double-pulse is interpreted in software as a single pulse. It's not ideal (hardware with one defect has a higher chance of also having other issues), but it's accurate.