I was lucky enough to get this double packaged Special Delivery Pikachu direct from the source! I’ve seen two others so far, one with the double white cards like mine has and one with only a single white card.
I wonder if other sealed promos have had this happen before 
That’s really cool and the first I’ve seen. I wonder if these promos are sealed by hand?
Nah
Pokémon gave you an extra $300, nice
They delivered.
That’s some good luck! Wonder what exactly caused this, I guess it’s worth even more as well because the “error”.
Very nice! Congrats on the find.
As for your question:
It does happen sometimes indeed, and can in theory happen with any sealed promo. I’ve seen some sealed WotC era promos that had two instead of one card as well, and I actually own a sealed WB Pikachu like that myself. 
Greetz,
Quuador
Huh. That’s actually really cool! Congrats on the extra!