Overlapping issue

Hy, I have just started to use Blender and came across this problem.

As shown in the picture I have an issue with overlapping. On the left plane two vertices are overlapping and I know it is because of triangulation. On the left is triangulated plane mesh and if we move vertex over the edge of triangle we wold see overlapping, but why it is happening this with quad faces.

You might want to do a forum search for “concave quads”. In short: Blender doesn’t like them.