Simple answer: When someone is granted an extra attack (ie, Leia, OR Senator, etc), the bonus applies only to the first attack, and any attack from Twin Attack is made as normal. When the damage-boost comes from something that applies to "all attacks this turn" or something (Sith Rage, or Bastilla's ABM, or Opportunist, etc), then the bonuses apply to all the attacks.
Example:
Let's pretend that Atton Jaq Rand would somehow gain access to Twin Attack (BAD idea, but just pretend)...
+9 for 20 is his regular attack
+4/+10 for Assassin
+10 dmg for Bastilla's ABM
So he's shooting every attack vs living targets at +13 attack for 40dmg while ABM is active.
And then let's pretend that the OR Senator gives him an attack (with the +10 dmg bonus) via his CE. In that case, Atton's first attack would be +13 for 50, while the Twin would be at +13 for 40.
Make sense?
[And BTW, I sincerely hope that Atton NEVER has access to Twin Attack.
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