2nd chance Darkrai now available for American region games
It cannot be obtained on the same game as the Darkrai from the earlier Pokémon 20th Anniversary distribution, and is identical to it.
This second chance distribution was originally announced to be available via Pokémon Trainer Club email, which would have meant a unique code was emailed to eligible players like was done for Mew for American region games. That code would have expired on November 30, 2016, instead.
A second chance Darkrai is also currently available for PAL region Pokémon X, Y, Omega Ruby, and Alpha Sapphire games until September 30, 2016 using the common serial code DARKRAI2016.