I remember fighting and shooting everything as Duke spouted one liners he borrowed from Bruce Campbell.
What prevented the dancing girls & Duke Nukem from morphing during crisises that made him our last, crass hope for mankind?
In Duke Nukem Forever, there is no mutation. The Aliens bring their pigs with them. In Duke Nukem: Manhattan Project there was this green mutation goo, that reduced your health. I'd say you need long exposure (especially being a big tough guy like the Duke) to the goo in order to mutate. As for "the chicks", we learn in Forever that the aliens require live women to breed their offspring, so it wouldn't make sense to mutate them.
I asked the co-creator of Duke Nukem; George Broussard this question on twitter.
His response is simply that Duke is too badass to mutate.
Q : "I'm struggling to work out why Duke isn't mutated by his own GLOPP gun. Any ideas?"
A : "You can't mutate [a] bad motherf*cker.