With cards like Sefris of the Hidden Ways or the new card Akki Battle Squad they have an additional line stating this ability triggers only once each turn. Will cards like Teysa Karlov and Wulfgar of Icewind Dale cause the ability to trigger an additional time even though it can only trigger once each turn? It doesn't say the ability can't trigger more than once each turn?
While I don't currently have any official rules on this, this is how I would rule on the situation currently as a Judge:
As a general rule in Magic: the Gathering, "can't" wins over "can", restrictions win over requirements or permission. In this case the "can't" is the triggers once per turn restriction on those cards and the "can" would be triggers an additional time. Because of this rule, the restriction is going to win out and prevent the ability triggering a second time. In the Comprehensive Rules, this "can't wins over must or can" is one of the Golden rules, rule 101.2:
101.2 When a rule or effect allows or directs something to happen, and another effect states that it can’t happen, the “can’t” effect takes precedence.
I created the following situation in Arena; on my battlefield I had (amongst other things):
- Teysa Karlov
- Dramatic Finale, Ghoulish Procession and Morbid Opportunist, three cards with a death trigger and the "only once each turn" wording.
- Archghoul of Thraben, Liliana, Dreadhorde General and Midnight Reaper, three cards with a death trigger but without the "only once each turn" wording.
- Fell Stinger (just as a target creature to die)
I then cast Infernal Grasp, targeting the Fell Stinger. I got only one copy of the Dramatic Finale, Ghoulish Procession and Morbid Opportunist triggers, but two copies of the Midnight Reaper, Liliana, and Archghoul of Thraben triggers, thus proving Teysa's ability was working.
Therefore at least with Teysa in Arena we can conclude the trigger is not doubled.