Single Purpose:

Use at the start of your activation. You may use the same special action up to twice during the activation.

does this mean that dark helmet can use brutality twice? as you are then performing two sets of attack (totally four attacks). Does it not super clash with the whole one attack a turn thingy

No you cannot Brutality twice with Vader but you could Force Choke twice.

cool, used it correctly then, thanks

Yup it still doesn't allow to to attack more than once but you can use any ability that is NOT an attack, like Force Choke. You can make that Elite Officer order Vader to attack twice, though...

You can make that Elite Officer order Vader to attack twice, though...

This. Ouch.