You got two models because that is what a mixin does. It fuses two codes into one. When you have a [ … ] every thing inside the list is a item. With a mixin or mixinto, you are just adding one more there. That is why you end up with two models.
In the old thread when I said the mixin replaces code, I was referring to the “text”:“your text here”. Only text and numbers are replaced. Everything else is added like in this case now, where it was not a text, but an item in a list.
The code show above by Hamnisu is not a mixin for the iconic. It is telling which file has the iconic, without blending them. If it was a mixin, the contents would fuse in that file and that would not work because the game expects it to be on its own file.
What you did was creating a full new file just for the iconic form of entity. It works, but was not needed.
Are you trying to change an iconic from a item already in the game that is not part of your mod?
Then you could simple override the model.
In the manifest: