This is the intended mechanic.
The decoys do not despawn when a new wave of decoys spawn. So if you do not kill the decoys, they will accumulate and most likely wipe you out.
What we typically do is stack all 4 of them together--the real one and the decoys--and we just focus the real one down and the decoys will die naturally from AoE cleave. And that is probably what Alcast does too. If they are not being stacked or if your AoE cleave damage is not high enough, then you will need to focus them down separately to kill them.