If opponent casts Falling Star and kills Karthus + a Deathknell unit with the same spell does the Deathknell still trigger twice?

Yes, the Deathknell unit's effect will trigger twice.

According to RiftJudge FAQ #5353, when Karthus and another unit with Deathknell die at the same time, the Deathknell unit's effect still triggers twice. This is because Karthus's ability is a passive ability, not a triggered ability.

Because it is a passive, it does not need to "evaluate" its own trigger condition at the moment of death; it simply applies its effect to the game state. Therefore, even though Karthus is leaving the board at the same time as the other unit, his effect remains active for that simultaneous event.

Summary of the interaction:

1. Falling Star resolves, dealing lethal damage to both Karthus and the other unit.
2. Both units are killed simultaneously.
3. Because Karthus's passive ability ("Your Deathknell effects trigger an additional time") is active at the moment of death, the other unit's Deathknell triggers once for its own death, plus one additional time due to Karthus.