In the book, Duke was kind of a racist.

It turned out being Aldas show which is why Trapper and Henry left. Helps having dad in high places
No offense, but you're an idiot.

I think he was also mentioned as being a brain surgeon that fell down a lot.

Tom Skerritt didn't want to be in it so they dropped the Duke character in the end.
In one episode though, he's mentioned as "that surgeon from Georgia that userd to be at the 4077th"

• Gothamite #4
  I've learned that it's OK to be flawed - Winona Ryder

That was certainly a weird reimagining of Duke's character: not only was he NOT friends with Hawkeye and Trapper, but he was also a klutz.

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Actually, you can explore a lot more character on a series than in a film. Sure, they're limited by the 30-minute runtime, but only in what they can explore in a
single episode
. With episode after episode after episode, you can explore a lot more characters than in a single two-hour film.
And as I've heard it, Spearchucker was written out when someone told the producers of the series that there were no black Army surgeons in the Korean theater.

They should have at least kept Ugly John. They had plenty of doctor characters, nurses, there was no need for a houseboy in a tent, but the 4077th still needed an anesthesiologist.
"You have to make choices with a half-hour sitcom."
I realize that, but you don't need to focus on the same characters over and over and over, do you? When Radar left, that's all they did. The same 7 characters. It would have been an ideal time to add at least 1 or two.

With a 30-minute (really less than 23 minutes with commercials) series, you needed to focus more on some characters and less on others. Ho-Jon, Ugly John, Spearchucker and Lt. Dish are examples of characters we see in the first episodes of the series but lose before Season 1 ended.
It's hard to imagine any focus on Cpt. Forrest with the need to establish and develop Hawkeye and Trapper John for the series, and I say that as someone who probably found Alan Alda at least as annoying as you. You have to make choices with a half-hour sitcom.