Why vtkCubeSource isn't a closed surface?

Hey All:
There are a question why the output of vtkCubeSource isn’t a closed one?
I write the codes to indicate my thinking. code below.

vtkNew《vtkCubeSource》 cube;
auto res = vtkSelectEnclosedPoints::IsSurfaceClosed(cube->GetOutput());

and the variable res is always equal to 0, which means the cube isn’t a closed surface.

It’s because vertices are duplicated to allow computation of normals (I believe). E.g.

from vedo import Cube
cube = Cube()
cube.clean()  # remove coincident vertices


Thans a lot, After i removed the duplicated points and the cube be a closed one.