I think that the discussion you linked is about two note types with the same name “Basic” but with different styling.
I’m getting the duplication issue even though the note types are exactly the same.
I’ve updated the original post to explain it better.
This is because the note types have different IDs, so Anki treats them as different regardless of whether their contents are identical.
Treating the two types as the same could prevent users from importing notes that have the same first fields as other notes already in the collection just because their note types happen to have identical contents (fields, names, styling, etc.), though their origins could be quite different.
This is admittedly quite easy to solve from the user’s side (e.g. making a trivial change to the templates of the existing type like adding a space so that the two types are treated as different),
@dae maybe Anki could employ a more complex note type comparison approach (after trying ID comparison) where the source type’s contents (including styling) are compared with the contents of all existing types to find a match? Not sure if this is practical or worth the effort.