[Rendar]

Attacking an ice/fire tornado (that isn't doubled up) causes you to take damage twice. See: Attacking a singular one of Relei's ice tornado's making you take full damage twice, as if you had walked onto it.

[Neus]

This is because attacking the tornado also causes it to apply its effect, so if you attack it while you're standing it, it will hit you twice. Just a weirdness of the skill.