Summon Reaper of Eviscerating Mechanics

Since the Summon Reaper of Eviscerating has a cast time of 1 second but the triggered ultimate attack has 1.5 second long attack time inherent to the minion, what happens if I cast the skill again during the 1.5 second attack time? Is the ultimate attack interrupted to start a new attack animation?

Did you ever find out the answer to this? Trying to figure this out myself for scaling viability.