The timestamp is the 32 Least Significant Bits (LSB) part of the 64-bit clock. Whenever the 32-bit portion of the clock rolls over, a _NTO_TRACE_CONTROLTIME control event is issued. Although adjacent events will never have the same exact timestamp, there may be some timestamp duplication due to the clock's rolling over.
The rollover control event includes the 32 Most Significant Bits (MSB), so you can reassemble the full clock time, if required. The timestamp includes only the LSB in order to reduce the amount of data being generated. (A 1-GHz clock rolls over every 4.29 seconds—an eternity compared to the number of events generated.)