In Adaptation, was Donald Kaufman a figment of Charlie's imagination (a la Fight Club) or was he a real character?

They looked pretty much exactly alike, yet Donald had the charisma and confidence to make friends and get a girlfriend, and he was also able to quickly write his screenplay while Charlie struggled. My wife, who loved the book, was upset with the ending and that he died in the end, and I argued that he wasn't even a real person in real life or in the movie.