"Most women need encouragement and companionship more than they need drugs" ~ Ina May Gaskin
Maybe your birth partner is confused about the role of a doula. Maybe they think there will be no need for one as they will be there, and they will be enough. To that, you can ask "What do you know about me when I'm giving birth?"
Now, here's the cool thing about the combination of a birth partner and a doula: They know you, and I know birth. It's the perfect combination.