hotshot, correct. the doubles from what i have so far are metallic james, old school diesel and pirate salty.
i have also bought the 4 different 3 packs, which are yet to arrive, and expect the duplicates to be: classic thomas, hero victor, spooky emily [all in the same pack], also classic diesel 10, racer gordon and classic dash. so 6 duplicates out of 12.
from what i can tell, the 8 packs and 3 packs contain no doubles between the two. the doubles occur with the individual trains, but that range also carries some which arent seen in the blister packs.
im not worried about having more than one of some engines... can always sell them or keep as spares.