When a creature is exiled with Nightmare Shepherd, does Teysa Karlov create two copies of the creature when dying?
You get two 2/2 zombie tokens and up to one 1/1 Doomed Dissenter token.

Both Doomed Dissenter's ability and Nightmare Shepherd's ability trigger twice on Doomed Dissenter's death (i.e. it moved from the battlefield to the graveyard).


  700.4. The term dies means “is put into a graveyard from the battlefield.”


These abilities are now on the stack. Dissenter's own ability will resolve, no matter what happens to the Doomed Dissenter card in the graveyard, and you will eventually get 2 zombie tokens.

When it comes to resolving Nightmare Shepherd's ability, assuming you choose to exile the Dissenter card with the first instance, you get a 1/1 Dissenter token. The second Shepherd ability will no longer have a Dissenter card to exile, so that instance does nothing. If you don't choose to exile the Dissenter with the first ability, you can still do so with the second.

There are no replacement effects involved with these cards. None of the abilities have the words "instead" or any other term required for identifying as a replacement effect.


  614.1a Effects that use the word “instead” are replacement effects. Most replacement effects use the word “instead” to indicate what events will be replaced with other events.