If a creature dying causes a triggered ability of a permanent you control to trigger, that ability triggers an additional time.
Whenever another nontoken creature you control dies, you may exile it. If you do, create a token that’s a copy of that creature, except it’s 1/1 and it’s a Nightmare in addition to its other types.
Then, I sacrifice my Doomed Dissenter:
When Doomed Dissenter dies, create a 2/2 black Zombie creature token.
I'm still not sure about replacement effects and whether or not the Doomed Dissenter in this case actually dies, when I choose to let it go into exile to copy it as a token. Because the demon's ability doesn't mention "instead", I think the human still "hits" the graveyard before going into exile. This would mean Teysa does trigger twice, creating two 2/2 Zombies. Does the Nightmare Shepherd also create two copies of the Doomed Dissenter?
This case confuses me, since I can only exile the creature once? But the death trigger still happens twice instead of once? Any clarification would be much appreciated.