Triangles affecting each other's fragment shading

[mikael-wants-to-visit-disneyland]

I am trying to get a cube rendered:

const CUBE_VERTICES: [Vertex; 8] = [
    Vertex {
        position: (0.5, 0.5, 0.5),
    }, // 0
    Vertex {
        position: (-0.5, 0.5, 0.5),
    }, // 1
    Vertex {
        position: (-0.5, -0.5, 0.5),
    }, // 2
    Vertex {
        position: (0.5, -0.5, 0.5),
    }, // 3
    Vertex {
        position: (0.5, 0.5, -0.5),
    }, // 4
    Vertex {
        position: (-0.5, 0.5, -0.5),
    }, // 5
    Vertex {
        position: (-0.5, -0.5, -0.5),
    }, // 6
    Vertex {
        position: (0.5, -0.5, -0.5),
    }, // 7
];

Lets take two of the triangles as an example:

const INDICES: [u16; 6] = [
    3, 0, 1, 
    1, 0, 4 
];

const NORMALS: [Normal; 2] = [
    Normal {
        normal: (0.0, 0.0, 1.0),
    },
    Normal {
         normal: (0.0, 1.0, 0.0),
    }
 ];

Fragment shader:

[[stage(fragment)]]
fn main([[location(0)]] normal: vec3<f32>) -> [[location(0)]] vec4<f32> {
    let light: vec3<f32> = vec3<f32>(0.0, -1.0, -1.0);
    let color: vec3<f32> = vec3<f32>(1.0);
    let brightness: f32 = -dot(normalize(normal), normalize(light));
    let dark_color: vec3<f32> = vec3<f32>(0.1) * color;
    let out_color = vec4<f32>(mix(dark_color, color, vec3<f32>(brightness)), 1.0);
    return out_color;
}

I have positioned the camera and light such that when I get the triangles rendered individually, one is very bright, the other is in shadow. However, when they are rendered together, for some reason their brightnesses change, and there is a gradient in them. What is going on? Any ideas for solving this?

[mikael-wants-to-visit-disneyland]

I found out that the issue was my assumption that the normals are those of the triangles. They are in fact supposed to be the normals of the vertices. There is thus unfortunately no option but to duplicate the vertices for each triangle, in order to get the hard edges.