HK Assassin Droids have the Targeting Computer ability, which lets them reroll 1 attack die.

The Targeting Computer Skirmish Upgrade Deployment Card also lets the unit reroll 1 attack die.

Can the two be combined, to reroll 2 attack dice?

(I know that, for example, you can't use two Explosive Weaponry Command Cards at once, nor can you take two Luke Skywalkers - one Hero and one Jedi. In both cases, it's because the things involved have the same name. So can you, on the same attack roll, use Targeting Computer from two different sources: the group's own abilities, plus an attachment?)

There are no restrictions in having two abilities with the same name on one figure. You can also have Targeting Computer on an elite Probe Droid. Then it would have two Targeting Computer abilities, and each can be used once per attack.

And you can have Combat Suit skirmish upgrade attachment with the elite Echo Base Troopers.

You just can't play a command card with the same name in the same timing instance, because it is considered trying to activate the same ability twice in the same timing instance. Activating abilities with the same effect from two different sources is fine.

And in the campaign and Inspiring Leadership you can have Order / Executive Order from class cards with Imperial Officers, giving them two Order / Executive Order special actions.