Darius can ult ekko twice

So i was just in a ranked game with ekko vs darius top lane.....now i got him pretty low as did he but everytime he ults and deals damage i used my ult to reverse said damage while dealing my own and out of nowhere he ults me again...now to my understanding his ult only resets if he kills a target and it shouldnt even reset all the way if its at rank 1....later in the game it happened three more times where i use my ult after he ults me, i took the damage, then ulting myself...shouldnt it go on cd if it wasnt a killing blow...can someone please explain this and if rito sees this address this if it shouldnt be this way

Thanks