This is technically correct - the reaper is told "do this dash attack" and then immediately told "do this dash attack". It can't do two actions at once, only one, so one of these "inputs" is wasted.
It's the same case as if you click once to use a skill, then immediately click a second time while starting that skill, you'l do the skill once.
It's the same case as if you click once to use a skill, then immediately click a second time while starting that skill, you'l do the skill once.