If you move a unit to a combat showdown and have Mask of Foresight, can Akali Rogue Assassin move a unit from combat and then when moved back in that unit trigger Mask of Foresight a second time?

Yes — in this exact line, Mask of Foresight can trigger a second time, because the unit left the combat entirely and its re-entry starts a new Combat Showdown.

How it works:

1. Your unit moves in alone → gains Attacker designation → Mask triggers (+1 Might).
2. Akali, Rogue Assassin's ability moves that unit to base. With no friendly unit left, the Showdown ceases to be staged — that combat is over (no Combat Cleanup since damage was never dealt).
3. If Akali is Empowered, the unit is readied. It then Standard Moves back to the battlefield → Contested applies again, enemy units are there → a new Combat is staged → a new Showdown begins.
4. The unit gains Attacker designation "for the first time" in this new combat, and if it's alone again, Mask triggers again.

Mask's trigger is limited to once per unit per combat (FAQ #5044, #5554), but a new combat resets that eligibility — even in the same turn.

Critical caveat: this only works if the first combat actually ends. If other friendly units remain at that battlefield, the Showdown continues, and a unit that leaves and re-enters during the same combat will not re-trigger Mask. Also, the re-entry move needs a ready unit (Empowered Akali readies it) or another effect like Ride the Wind, since you can't Standard Move during a Showdown.