If I kill a Ruined Rex with a Baited Hook and from the Baited I play a Karthus the Deathknell from the Ruined also duplicates?

No, the Deathknell effect from Ruined Rex will not trigger twice in this scenario.

According to the established game rules and RiftJudge FAQs, Karthus, Eternal must already be on the battlefield at the exact moment a Deathknell trigger is created for his passive ability ("Your Deathknell effects trigger an additional time") to apply.

Sequence of Resolution:

1. Baited Hook is activated and begins resolving.
2. Ruined Rex is killed. Because it has Deathknell, its trigger is created and becomes pending. It cannot enter the chain yet because Baited Hook is still in the middle of resolving its instructions (Rule 734.1.d.2 / Rule 808.1.d.2).
3. Baited Hook continues resolving: You look at the top 5 cards, find Karthus, Eternal, and play him. He becomes a pending item on the chain.
4. Baited Hook finishes resolving. Now, all pending items finalize in the order they were created:
   • First, the Deathknell trigger for Ruined Rex finalizes and enters the chain.
   • Second, Karthus, Eternal finalizes and enters the battlefield.

Because the Deathknell trigger was created and placed on the chain before Karthus was on the battlefield, his passive ability does not see the event to double it. You will only get the single, base Deathknell trigger.

This ruling is consistent with multiple RiftJudge FAQs (including #10051, #1116, #9256, and #8551), which confirm that Karthus must be present on the board when the trigger is created, not just when it resolves.