robus
Lowcountry Low Roller
'Hits' refers to a successful attack roll. Fireball doesn't use attack rolls, therefore the fire damage does not 'hit' anything at all! Since the duplicates were not 'hit' by an 'attack', they ignore the fire damage and are unaffected by the fireball.
You're probably right - but it's not much fun (and seems awfully nit-picky for a free-flowing game). Fun will win over strict adherence to the rules at my table I guess.