If Baited Hook kills Machine Evangel and plays Karthus as part of its resolution, does Karthus's ability cause Machine Evangel's Deathknell to trigger twice?

Ruling: No, Karthus does not cause Machine Evangel's Deathknell to trigger twice.

Sequence:

• Baited Hook's activated ability goes on the chain
• During Baited Hook's resolution, Machine Evangel is killed
• Both the Deathknell trigger and Karthus being played wait until after Baited Hook finishes resolving to enter the chain
• Since they enter the chain at the same time (or Deathknell enters before Karthus), Karthus is not on board when the Deathknell triggers
• Karthus doubles triggers, not resolutions, so the Deathknell only triggers once

Nuances:

• Things cannot enter the chain during the resolution of another effect; they wait until after resolution completes
• The exact ordering of simultaneous chain entries (Karthus and Deathknell) is still unclear and awaiting official clarification
• This ruling may change in future Comprehensive Rules updates, as the interaction of "play" effects with the chain is still being finalized by Riot