Does Karthus's effect stack if multiple copies are active, causing death-knell effects to trigger more than twice?

Ruling: Yes, multiple copies of Karthus stack. Each Karthus adds one additional trigger to death-knell effects.

Sequence:

• Base death-knell effect triggers once
• Each active Karthus adds +1 additional trigger
• With 2 Karthus copies: death-knell triggers 3 times total (1 base + 1 per Karthus)