Given that the rules say a token cannot be spent twice during an attack, why does the ISD with boarding troopers need the Avenger title? Am I missing a rule here but doesn't Avenger prevent the defender doing something the rules already say it can't do?

Boarding Troopers does not take place during an attack.

Boarding Troopers happens when you reveal a command dial, at which point it takes effect. The attack, and so the Avenger effect, happen later.

So, reveal dial, use troopers, targets tokens are now spent. When you come around to attacking (usually right after) the target is now defenseless, thanks to Avenger.

Ah yes, I see. A token can't be spent twice during an attack, I got that right but Boarding Troopers do their thing before the attack starts, ie in the reveal command part of the sequence. Thanks for helping me out with that.