OpenGL primitives rendered twice

Hello

I want to create health bar for my game using openGL primitives (I'm new in Unity, but have great experience in openGL). I used this code :

void OnGUI()
{
GL.Begin(GL.TRIANGLE_STRIP);
GL.Color(new Color(1, 1, 0, 1.0F));
GL.Vertex3(10, 10, 0);
GL.Vertex3(50, 10, 0);
GL.Vertex3(10, 50, 0);
//GL.Vertex3(500, 500, 0);
GL.End();
}

in player class. It display me TWO yellow triangles (look at posted picture). However, wanted to make screenshot I discovered that, when I turn off Canvas (or everything in Canvas) it display everything correctly (another thing that when everything is turned off, lightning affect my poor triangle). I could stay in that point, but in nearby future I need to add more GUI elements.

I'm writing again, I'm new to Unity, so I could done something wrong.

Thanks for reading this post and I please all of you for help