Graves Bug ! 2 attacks in one [GAMEPLAY]

Hey, I was playing a lot graves these times, and found that we could reset attacks extremely fast with E. So I trained while farming jungle and did it in a ranked game. That seems very gamebreaker to me, and I don't know if that's normal. So before tryharding practice tool, I'd like to know if that's "legal". Here's the perfect exemple, where ekko did not have time to ult, as the damage was really insane.