I tend to agree with Duo --- I usually hear "on the list" when people speak of lists
I think/use "on the list" usually but in python programming I tend to think "in the list" when dealing with lists. Probably because a list in a computer program you often do not have a snap shot of it in your mind. The contents are often ambiguous. You have the "in" operator in python to test if an item is in/on a list too.
I would use it is one of the things in the list as well as on the list. I think both are correct and both should be accepted.
In everyday English "in the list" will sound strange for most native speakers. It's not necessarily wrong, but it's the kind of thing that would make the old-timers say, "You ain't from around here, are you?"
From upstate New York, "in the list" is most definitely the more common way of saying this. It's what I put, and was surprised when Duo flagged it wrong. DL should accept both of these as correct, I'd think.
When I hear "on the list," I imagine a conversation like this. "Where is the grocery list?" "I put it under the keys so you won't forget it." "Oh, so the keys are ON the list."
You aren't buying new keys as part of the grocery list. You buy the things listed IN the list.
Native speaker, and I asked around. Everyone I talked to IN Indiana (not ON Indiana) said IN the list.