The 10th mission the Duke gives you the dialog is as following;
"My sister has a birthday coming up, and she doesn't feel good about it. She feels like she's getting old, but I can't exactly do nothing about that. It's, what's the word... inevitabottle. Yeah, "inevitable", like I said. Last year I bought her some of those creams for stopping wrinkles, but she just cried all day when I gave it her."
Right at the end, the Duke should say ^ "give it to her."
Not an error. That's how Duke speaks. If you didn't notice it before, 'inevitabottle' should be enough of a clue.
as above, its intentional
If it was "How the Duke speaks," "Inevitabottle" wouldn't have been corrected during the dialog. Its an error that doesn't read right. I've read many of Dukes posts and he doesn't tend to talk with these errors and when he does its corrected, by himself. So please, fix YOUR grammar instead of blaming it on the NPC you created.
This is how its meant to be, duke is a character in a fictional game, his character doesnt use correct English.
Obviously, you haven't read enough of his posts, or haven't read them well enough. He only corrects himself when the player (or rather, the person the player is roleplaying) corrects him. I gave you one example of his mistakes from the mission you quoted, but if you're too lazy to look for more, by all means, let me help:
"I don't care what favor you just done for me, that was work."
"This ain't no fairytale and my sister is no damsel in this dress."
"You shaved my bacon there pal"
"You ain't told anyone what was in the safe though have you?"
Did that convince you or should I go through Duke's forum posts as well?