National Instruments NI USB-621x - Manual (Page 118)

Chapter 9 Counters
NI USB-621x User Manual 9-38 ni.com
Even if the Source pulses are long, the counter increments only once for
each Source pulse.
Normally, the counter value and Counter n Internal Output signals change
synchronously to the Source signal. With duplicate count prevention, the
counter value and Counter n Internal Output signals change synchronously
to the 80 MHz Timebase.
Enabling Duplicate Count Prevention in NI-DAQmx
Duplicate count prevention is automatically used with the USB-621x
devices. Disabling duplicate count prevention is not supported.