Flat shading with vertex arrays

I have generated my normals in the following fashion:

		CVector Edge_01 = m_Model->Vertices[Point1] - m_Model->Vertices[Point0];
		CVector Edge_12 = m_Model->Vertices[Point2] - m_Model->Vertices[Point1];
		CVector Edge_20 = m_Model->Vertices[Point0] - m_Model->Vertices[Point2];
		CVector Normal = Edge_01.Cross(Edge_12);

		m_Model->Normals[Point0] += ACos(Edge_20.Dot(Edge_01)) * Normal;
		m_Model->Normals[Point1] += ACos(Edge_01.Dot(Edge_12)) * Normal;
		m_Model->Normals[Point2] += ACos(Edge_12.Dot(Edge_20)) * Normal;

When smooth shading these appear as they should however when I switch the shading mode to flat, boxy objects appear to have every second triangle blackā€¦

Any thoughts?

Many thanks